Image forming lens system and image pickup apparatus including the same

ABSTRACT

An image forming lens system includes a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit. The first lens unit includes a front-side lens unit having a positive refractive power and a rear-side lens unit in order from the object side. The second lens unit moves at a time of focusing. The third lens unit includes a positive lens element. Each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a). The rear-side lens unit includes a negative lens element and a positive lens element. The front-side lens unit includes a diffractive lens component having a diffractive lens surface. The diffractive lens component has a positive refractive power. Following Conditional Expression (1) is satisfied:
 
−0.7≤ f/f Lens  (a)
 
0.015≤Δ GFGR/f ≤0.25  (1).

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2016-243856 filed on Dec. 15, 2016; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention Description of the Related Art

The present invention relates to an image forming lens system and an image pickup apparatus including the same.

Photography using a telephoto lens or a super-telephoto lens (hereinafter referred to as “telephoto lens”) has the effect of making a distant subject or a small subject appear in front of the photographer. For this reason, telephoto lenses are widely used in various scenes such as taking photographs of sports scenes, taking photographs of wild animals such as wild birds, and astrophotography.

Examples of the telephoto lenses for use in taking photographs of such scenes are disclosed in Japanese Patent Application Laid-open Nos. 2009-139543 (first example), 2008-261969 (third example), 2013-250293 (third example), and 2012-145789 (first example).

SUMMARY OF THE INVENTION

An image forming lens system of the present invention comprises:

in order from an object side to an image side,

a first lens unit having a positive refractive power;

a second lens unit having a negative refractive power; and

a third lens unit,

wherein

the first lens unit includes in order from the object side, a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween,

the second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing,

the third lens unit includes a positive lens element,

each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a),

the rear-side lens unit includes a negative lens element and a positive lens element,

the front-side lens unit includes a diffractive lens component having a diffractive lens surface,

the diffractive lens component has a positive refractive power, and

following Conditional Expression (1) is satisfied: −0.7≤f/fLens  (a) 0.015≤ΔGFGR/f≤0.25  (1)

where

f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

fLens is a focal length of each lens element in the front-side lens unit,

ΔGFGR is an on-axis air space from an image-side surface in the front-side lens unit to an object-side surface in the rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface, and having no other effective transmission surface between the two effective transmission surfaces.

Furthermore, another image forming lens system of the present invention comprises:

in order from an object side to an image side,

a first lens unit having a positive refractive power;

a second lens unit having a negative refractive power; and

a third lens unit,

wherein

the first lens unit includes in order from the object side, a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween,

the second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing,

the third lens unit includes a positive lens element,

each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a),

the rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit,

the first rear-side lens unit includes a negative lens element and a positive lens element,

the second rear-side lens unit includes a positive lens element,

the front-side lens unit includes a diffractive lens component having a diffractive lens surface,

the diffractive lens component has a positive refractive power, and

following Conditional Expression (2) is satisfied: −0.7≤f/fLens  (a) 0.1≤ΔGFFGR1/f≤0.5  (2)

where

f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

fLens is a focal length of each lens element in the front-side lens unit,

ΔGFFGR1 is an on-axis distance from an object-side surface in the front-side lens unit to an object-side surface in the first rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface, and having no other effective transmission surface between the two effective transmission surfaces.

Furthermore, another image forming lens system of the present invention comprises:

in order from an object side to an image side,

a first lens unit having a positive refractive power;

a second lens unit having a negative refractive power; and

a third lens unit,

wherein

the first lens unit includes in order from the object side, a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween,

the second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing,

the third lens unit includes a positive lens element,

the rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit,

the first rear-side lens unit includes a negative lens element and a positive lens element,

the second rear-side lens unit includes a positive lens element,

the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface,

the diffractive lens component is disposed closest to the image side in the front-side lens unit and includes a diffractive lens element closest to the image side, and following Conditional Expression (1-1) is satisfied: 0.015≤ΔGFGR1Do/f≤0.3  (1-1)

where

f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

ΔGFGR1Do is an on-axis air space from an image-side surface of the diffractive lens element to an object-side surface in the first rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface, and having no other effective transmission surface between the two effective transmission surfaces.

Furthermore, another image forming lens system of the present invention comprises:

in order from an object side to an image side,

a first lens unit having a positive refractive power;

a second lens unit having a negative refractive power; and

a third lens unit,

wherein

the first lens unit includes, in order from the object side, a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween,

the second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing,

the third lens unit includes a positive lens element,

each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a),

the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface,

the diffractive lens component is disposed closest to the image side in the front-side lens unit and includes a diffractive lens element closest to the image side, and

following Conditional Expression (1-2) is satisfied: −0.7≤f/fLens  (a) 0.02≤ΔGFGRDo/f≤0.3  (1-2)

where

f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

fLens is a focal length of each lens element in the front-side lens unit,

ΔGFGRDo is an on-axis air space from an image-side surface of the diffractive lens element to an object-side surface in the rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface, and having no other effective transmission surface between the two effective transmission surfaces.

Furthermore, another image forming lens system of the present invention comprises:

in order from an object side to an image side,

a first lens unit having a positive refractive power;

a second lens unit having a negative refractive power; and

a third lens unit,

wherein

the first lens unit includes, in order from the object side, a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween,

the second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing,

the third lens unit includes a positive lens element,

the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface,

the diffractive lens component is disposed closest to the image side in the front-side lens unit,

the rear-side lens unit includes a positive lens and a negative lens,

an air space between the diffractive lens component and the rear-side lens unit is largest of air spaces in the first lens unit,

the image forming lens system further comprises an aperture stop disposed closer to the object side than the second lens unit and restricts on-axis luminous flux, and

following Conditional Expression (1-3) is satisfied: 0.01≤ΔGFDoGRmax/f≤0.2  (1-3)

where

f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

ΔGFDoGRmax is a largest on-axis air space of on-axis air spaces from an image-side surface of the diffractive lens component to an object-side surface in the rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface, and having no other effective transmission surface between the two effective transmission surfaces.

Furthermore, an image pickup apparatus of the present invention comprises:

an optical system; and

an image pickup element having an image pickup surface and converting an image formed on the image pickup surface by the optical system into an electrical signal,

wherein

the optical system is any one of the image forming lens systems described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are lens cross-sectional diagrams of an image forming lens system of Example 1;

FIG. 2A and FIG. 2B are lens cross-sectional diagrams of an image forming lens system of Example 2;

FIG. 3A and FIG. 3B are lens cross-sectional diagrams of an image forming lens system of Example 3;

FIG. 4A and FIG. 4B are lens cross-sectional diagrams of an image forming lens system of Example 4;

FIG. 5A and FIG. 5B are lens cross-sectional diagrams of an image forming lens system of Example 5;

FIG. 6A and FIG. 6B are lens cross-sectional diagrams of an image forming lens system of Example 6;

FIG. 7A, FIG. 7B, FIG. 7C, FIG. 7D, FIG. 7E, FIG. 7F, FIG. 7G, and FIG. 7H are aberration diagrams of the image forming lens system of Example 1;

FIG. 8A, FIG. 8B, FIG. 8C, FIG. 8D, FIG. 8E, FIG. 8F, FIG. 8G, and FIG. 8H are aberration diagrams of the image forming lens system of Example 2;

FIG. 9A, FIG. 9B, FIG. 9C, FIG. 9D, FIG. 9E, FIG. 9F, FIG. 9G, and FIG. 9H are aberration diagrams of the image forming lens system of Example 3;

FIG. 10A, FIG. 10B, FIG. 10C, FIG. 10D, FIG. 10E, FIG. 10F, FIG. 10G, and FIG. 10H are aberration diagrams of the image forming lens system of Example 4;

FIG. 11A, FIG. 11B, FIG. 11C, FIG. 11D, FIG. 11E, FIG. 11F, FIG. 11G, and FIG. 11H are aberration diagrams of the image forming lens system of Example 5;

FIG. 12A, FIG. 12B, FIG. 12C, FIG. 12D, FIG. 12E, FIG. 12F, FIG. 12G, and FIG. 12H are aberration diagrams of the image forming lens system of Example 6;

FIG. 13 is a cross-sectional view of an image pickup apparatus;

FIG. 14 is a front perspective view of the image pickup apparatus;

FIG. 15 is a rear perspective view of the image pickup apparatus; and

FIG. 16 is a configuration block diagram of an internal circuit of the main part of the image pickup apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Prior to a description of examples, the operation effects of embodiments according to an aspect of the present invention will be described. When the operation effects of the present embodiment are described, specific examples will be illustrated. However, as is the case with the examples described later, the illustrated aspects are only a few of the aspects included in the present invention, and the aspects include a number of variations. Therefore, the present invention is not limited to the illustrated aspects.

An image forming lens system of the present embodiment includes a common configuration. In the common configuration, the optical system includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit. The first lens unit includes in order from the object side, a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween. The second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing. The third lens unit includes a positive lens element.

In order to reduce the overall length of the optical system and to ensure favorable imaging performance from the center to the periphery of an image, it is important to ensure optical symmetry in the entire optical system. In the first common configuration, the optical system is constructed with a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive lens element.

In this case, with the first lens unit, the second lens unit, and the positive lens element in the third lens unit, the refractive powers are disposed in the order of: positive refractive power, negative refractive power, and positive refractive power. That is, refractive powers are in symmetric arrangement. In this way, by adopting the arrangement as described above, in the common configuration, since the optical symmetry can be ensured, it becomes easy to correct coma, distortion, and chromatic aberration of magnification favorably.

In the first lens unit, the front-side lens unit is disposed closest to the object side, and the rear-side lens unit is disposed on the image side of the front-side lens unit with an air space that is wide to some degree. The front-side lens unit has a positive refractive power. Hence, for example, by providing the rear-side lens unit with negative refractive power, it is possible to perform correction of spherical aberration and correction of chromatic aberration.

Furthermore, telephoto action is produced by a combination of positive refractive power and negative refractive power. In the first lens unit, it is possible to enhance telephoto action by the positive refractive power of the front-side lens unit and the negative refractive power of the rear-side lens unit. As a result, the total length of the optical system can be reduced.

The second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing.

The second lens unit is located midway between the first lens unit and the third lens unit and has a negative refractive power. In the common configuration, the second lens unit is set as a focus lens unit, and focusing is performed by the second lens unit. By doing so, it becomes easy to reduce the weight of the focus lens unit and to ensure favorable imaging performance on the periphery of the image at a time of focusing.

An image forming lens system of a first embodiment includes the common configuration. In addition, each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a), the rear-side lens unit includes a negative lens element and a positive lens element, the front-side lens unit includes a diffractive lens component having a refractive lens surface, the diffractive lens component has a positive refractive power, and following Conditional Expression (1) is satisfied: −0.7≤f/fLens  (a) 0.015≤ΔGFGR/f≤0.25  (1)

where

f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

fLens is a focal length of each lens element in the front-side lens unit,

ΔGFGR is an on-axis air space from an image-side surface in the front-side lens unit to an object-side surface in the rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface, and having no other effective transmission surface between the two effective transmission surfaces.

In the image forming lens system of the first embodiment, the front-side lens unit includes a diffractive lens component having a diffractive lens surface. The diffractive lens component has a positive refractive power.

An example of optical elements for correcting chromatic aberration, spherical aberration, and the like is a diffractive optical element (DOE). The diffractive optical element has a diffractive surface. In the diffractive optical element, an optical surface other than the diffractive surface can be formed as a refractive surface. In this case, the diffractive optical element can be used as a lens element or a lens component.

The diffractive lens component is a lens component in which a diffractive optical element is used as the lens component. The diffractive lens component has a diffractive lens surface.

If the light flux diameter incident on the diffractive optical element is large, it is possible to favorably correct aberration with smaller refractive power. In the optical system having the common configuration, the on-axis light flux diameter is large in the front-side lens unit.

Based on this, in the image forming lens system of the first embodiment, the diffractive lens component is disposed in the front-side lens unit. By doing this, it becomes possible to favorably correct chromatic aberration, spherical aberration, and the like with smaller refractive power. As a result, it is possible to effectively achieve size reduction and weight reduction of the optical system.

In the optical system having the common configuration, the on-axis light flux diameter is largest at a position closest to the object side in the front-side lens unit or in the vicinity thereof. Thus, it is preferable that the diffractive lens component be disposed closest to the object side in the front-side lens unit or in the vicinity thereof.

In a diffractive optical element, the value of Abbe number is −3.45 (νd=−3.45). Therefore, the diffractive optical element has negative dispersion.

Then, a diffractive optical element is used as a positive lens element in the front-side lens unit. That is, the refractive power of the diffractive lens component is set to a positive refractive power. By doing this, it becomes possible to favorably correct chromatic aberration even without disposing a negative lens element in the front-side lens unit.

Moreover, since the refractive power of the positive lens element can be increased, it is possible to favorably correct chromatic aberration. Moreover, since the volume of a diffractive optical element is small, it is also possible to achieve weight reduction of the diffractive lens component.

It is possible to provide the diffractive lens component with the same effect as the correction effect by an aspherical surface simultaneously with the correction effect for chromatic aberration. Therefore, it is possible to reduce the amount of spherical aberration produced in the front-side lens unit even when the positive refractive power in the front-side lens unit is increased.

The rear-side lens unit includes a negative lens element. Thus, telephoto action is obtained with the front-side lens unit and the negative lens element in the rear-side lens unit. If the positive refractive power in the front-side lens unit can be increased, it is possible to enhance the telephoto action. As a result, it becomes possible to further reduce the total length of the optical system.

Since the refractive power of the front-side lens unit is a positive refractive power, the front-side lens unit includes a positive lens element. The correction of chromatic aberration by the diffractive lens component becomes possible whereby the choices of the Abbe number of the positive lens element are increased in the front-side lens unit. Since the choices of the Abbe number are increased, it also becomes possible to form the positive lens element with a glass material having relatively low density. As a result, it is possible to achieve further weight reduction of the optical system.

In the front-side lens unit, it is possible to dispose a negative lens element. In this case, it is preferable that the negative lens element disposed in the front-side lens unit be a lens element that satisfies Conditional Expression (a).

The volume of the front-side lens unit is largest in the optical system. Hence, in order to reduce the weight of the optical system, it is preferable that a lens element having a large weight be not disposed in the front-side lens unit.

In the image forming lens system of the first embodiment, it is possible to dispose a negative lens element in the front-side lens unit. A negative lens element has a larger weight as the refractive power increases. Hence, when a negative lens element is disposed in the front-side lens unit, it is preferable that a negative lens element that satisfies Conditional Expression (a) be disposed.

In the negative lens element that satisfies Conditional Expression (a), since the negative refractive power is small, the weight of the lens element is not likely to increase. Hence, it is possible to adopt a configuration in which a negative lens element having a large weight, that is, a negative lens element having large refractive power is not disposed in the front-side lens unit with the largest volume. As a result, it is possible to significantly reduce the weight of the optical system.

A negative lens element that exceeds the lower limit value of Conditional Expression (a) is a lens element having small refractive power. Therefore, even when a negative lens element is disposed in the front-side lens unit, the volume of the lens is not increased as long as the lens element that exceeds the lower limit value of Conditional Expression (a).

Furthermore, since the refractive power is small, even when such a lens element is disposed in the front-side lens unit, it is possible to ensure positive refractive power necessary for the front-side lens unit, with no problem. Hence, it is acceptable that a lens element that exceeds the lower limit value of Conditional Expression (a) is disposed in the front-side lens unit.

Examples of the lens element that exceeds the lower limit value of Conditional Expression (a) include cover glass having a small refractive power, a lens with a thin resin layer having aspherical effects on the lens surface (HBL: hybrid lens), and an optical element having a diffraction-effect surface (DOE: diffractive optical element).

Although the negative lens element that satisfies Conditional Expression (a) may not necessarily be a separator between the front-side lens unit and the rear-side lens unit, additionally, a configuration may be made such that all the lenses closer to the object side than the negative lens closest to the object side of the negative lenses that satisfy Conditional Expression (a) are included in the front-side lens unit.

By satisfying Conditional Expression (1), it is possible to ensure a sufficient space between the front-side lens unit and the rear-side lens unit. In this case, since the front-side lens unit and the rear-side lens unit can be separated from each other, it is possible to enhance telephoto action in the first lens unit. As a result, it is possible to reduce the overall length of the optical system. It is further possible to reduce the weight of the lenses positioned closer to the image side than the front-side lens unit.

In a case of falling below a lower limit value of Conditional Expression (1), since the telephoto action in the first lens unit is weakened, it becomes difficult to reduce the overall length of the optical system. To reduce the overall length of the optical system, the telephoto action may be enhanced with the first lens unit and the second lens unit. However, if doing so, since the optical symmetry is deteriorated, it difficult to ensure favorable imaging performance. In addition, since the weight of the negative lens in the rear-side lens unit is increased, it becomes difficult to make the optical system light weight.

In a case of exceeding an upper limit value of Conditional Expression (1), since the overall length of the first lens unit is increased, it becomes difficult to reduce the overall length of the optical system.

It is preferable that following Conditional Expression (a′), (a″), or (a′″) be satisfied instead of Conditional Expression (a): −0.6≤f/fLens  (a″) −0.5≤f/fLens  (a′) −0.4≤f/fLens  (a′″).

It is preferable that following Conditional Expression (1′), (1″), (1′″), or (1″″) be satisfied instead of Conditional Expression (1): 0.02≤ΔGFGR/f≤0.22  (1′) 0.03≤ΔGFGR/f≤0.2  (1″) 0.04≤ΔGFGR/f≤0.17  (1′″). 0.05≤ΔGFGR/f≤0.16  (1′″).

An image forming lens system of a second embodiment has the common configuration. In addition, each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a), the rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit, the first rear-side lens unit includes a negative lens element and a positive lens element, the second rear-side lens unit includes a positive lens element, the front-side lens unit includes a diffractive lens component having a diffractive lens surface, the diffractive lens component has a positive refractive power, and following Conditional Expression (2) is satisfied: −0.7≤f/fLens  (a) 0.1≤ΔGFFGR1/f≤0.5  (2)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

fLens is the focal length of each lens element in the front-side lens unit,

ΔGFFGR1 is an on-axis distance from an object-side surface in the front-side lens unit to an object-side surface in the first rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface, and having no other effective transmission surface between the two effective transmission surfaces.

In the image forming lens system of the second embodiment, the front-side lens unit includes a diffractive lens component in the same manner as in the image forming lens system of the first embodiment. Thus, the operation effect described in the image forming lens system of the first embodiment can be obtained.

In the image forming lens system of the second embodiment, the rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit. The first rear-side lens unit includes a negative lens element and a positive lens element. The second rear-side lens unit includes a positive lens element.

The front-side lens unit is located closest to the object side in the first lens unit. The front-side lens unit is made to be composed of a plurality of lens elements having a positive refractive power alone, and the first rear-side lens unit is made to be disposed on the image side with an air space interposed. Then, a negative lens element and a positive lens element are made to be disposed in the first rear-side lens unit.

By doing so, the first lens unit has a telephoto-type optical system. As a result, it is possible to reduce the total length of the optical system.

Furthermore, a negative lens element with a larger volume is not disposed in the front-side lens unit, whereby it is possible to significantly reduce the weight of the optical system.

The second rear-side lens unit is disposed on the image side of the first rear-side lens unit with an air space interposed. A positive lens element is made to be disposed in the second rear-side lens unit. In this case, the arrangement of refractive power in the first lens unit becomes positive refractive power, negative refractive power, and positive refractive power. That is, refractive power is symmetrically arranged. In this manner, since optical symmetry is ensured in the first lens unit, it is possible to enhance the correction effect for spherical aberration, astigmatism, longitudinal chromatic aberration, and chromatic aberration of magnification.

Since it is possible to enhance the correction effect in the first lens unit, it becomes possible to reduce occurrence of aberration in the optical system located closer to the image side than the first lens unit. Hence, even when the total length of the optical system is reduced, it is easier to reduce the weight of the lens system located closer to the image side than the first lens unit and to ensure the performance at a time of focusing.

The technical meaning of Conditional Expression (a) is as described above.

By satisfying Conditional Expression (2), it is possible to ensure a sufficient space between the front-side lens unit and the first rear-side lens unit. In this case, since the front-side lens unit and the first rear-side lens unit can be separated from each other, it is possible to enhance telephoto action in the first lens unit. As a result, it is possible to reduce the overall length of the optical system. It is further possible to reduce the weight of the lenses positioned closer to the image side than the front-side lens unit.

In a case of falling below a lower limit value of Conditional Expression (2), since the telephoto action in the first lens unit is weakened, it becomes difficult to reduce the overall length of the optical system. To reduce the overall length of the optical system, the telephoto action may be enhanced with the first lens unit and the second lens unit. However, if doing so, since the optical symmetry is deteriorated, it difficult to ensure favorable imaging performance. In addition, since the weight of the negative lens in the first rear-side lens unit is increased, it becomes difficult to make the optical system light weight.

In a case of exceeding an upper limit value of Conditional Expression (2), since the overall length of the first lens unit is increased, it becomes difficult to reduce the overall length of the optical system.

It is preferable that following Conditional Expression (2′) or (2″) be satisfied instead of Conditional Expression (2): 0.12≤ΔGFFGR1/f≤0.45  (2′) 0.13≤ΔGFFGR1/f≤0.4  (2″).

An image forming lens system of a third embodiment has the common configuration. In addition, the rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit, the first rear-side lens unit includes a negative lens element and a positive lens element, the second rear-side lens unit includes a positive lens element, the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface, the diffractive lens component is disposed closest to the image side in the front-side lens unit and includes a diffractive lens element closest to the image side, and following Conditional Expression (1-1) is satisfied: 0.015≤ΔGFGR1Do/f≤0.3  (1-1)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

ΔGFGR1Do is an on-axis air space from an image-side surface of the diffractive lens element to an object-side surface in the first rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface, and having no other effective transmission surface between the two effective transmission surfaces.

In the image forming lens system of the third embodiment, the rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit, the first rear-side lens unit includes a negative lens element and a positive lens element, and the second rear-side lens unit includes a positive lens element, in the same manner as in the image forming lens system of the second embodiment. Thus, the operation effect described in the image forming lens system of the second embodiment can be obtained.

In the image forming lens system of the third embodiment, the front-side lens unit includes a diffractive lens component in the same manner as in the image forming lens system of the first embodiment. Thus, the operation effect described in the image forming lens system of the first embodiment can be obtained.

It is noted, however, that the refractive power of the diffractive lens component is not limited to a positive refractive power in the image forming lens system of the third embodiment. When the refractive power of the diffractive lens component is a positive refractive power, the operation effect described in the image forming lens system of the first embodiment can be obtained.

In the image forming lens system of the third embodiment, the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface, and the diffractive lens component is disposed closest to the image side in the front-side lens unit and includes a diffractive lens element closest to the image side.

When light is applied to a diffractive optical element, diffracted light with the design order term is emitted from the diffractive optical element. It is noted, however, that when light with high brightness is applied to a diffractive optical element, flare occurs. The order term of the flare is different from the design order term.

As described above, in the optical system having the common configuration, the on-axis light flux diameter is largest at a position closest to the object side in the front-side lens unit or in the vicinity thereof. Thus, it is preferable that the diffractive lens component be disposed closest to the object side in the front-side lens unit or in the vicinity thereof. By doing this, it becomes possible to favorably correct chromatic aberration, spherical aberration, and the like with smaller refractive power.

It is noted, however, that the position closest to the object side in the front-side lens unit or the vicinity thereof is irradiated with incident light from broad directions. Therefore, if a diffractive lens component is disposed at this position, the amount of flare produced is increased by light flux incident from a range other than the shooting range.

In the image forming lens system of the third embodiment, the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface, and the diffractive lens component is disposed closest to the image side in the front-side lens unit.

In this way, it is possible to keep the diffractive lens component away from the position closest to the object side in the front-side lens unit or the vicinity thereof. In this case, since a lens frame is provided closer to the object side than the diffractive lens component, it is possible to shield light flux responsible for flare with the lens frame. That is, light flux responsible for flare is not incident on the diffractive lens component. As a result, it is possible to reduce flare.

In addition, in the front-side lens unit of the image forming lens system of the third embodiment, the diffractive lens component includes a diffractive lens element closest to the image side.

By doing this, it is possible to keep the diffractive lens element away from the position closest to the object side in the front-side lens unit or the vicinity thereof. In this case, since a lens frame is provided closer to the object side than the diffractive lens element, it is possible to shield light flux responsible for flare with the lens frame. That is, light flux responsible for flare is not incident on the diffractive lens element. As a result, it is possible to reduce flare.

Moreover, since a diffractive lens component is included in the front-side lens unit, it is possible to achieve the correction effect for chromatic aberration.

The technical meaning of Conditional Expression (a) is as described above.

The technical meaning of Conditional Expression (1-1) is the same as the technical meaning of Conditional Expression (2).

It is preferable that following Conditional Expression (1-1′), (1-1″), (1-1′″), (1-1″″), or (1-1′″″) be satisfied instead of Conditional Expression (1-1): 0.02≤ΔGFGR1Do/f≤0.27  (1-1′) 0.03≤ΔGFGR1Do/f≤0.25  (1-1″) 0.04≤ΔGFGR1Do/f≤0.5  (1-1′″) 0.05≤ΔGFGR1Do/f≤0.16  (1-1″″) 0.07≤ΔGFGR1Do/f≤0.15  (1-1′″″).

An image forming lens system of a fourth embodiment has the common configuration. In addition, each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a), the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface, the diffractive lens component is disposed closest to the image side in the front-side lens unit and includes a diffractive lens element closest to the image side, and following Conditional Expression (1-2) is satisfied: −0.7≤f/fLens  (a) 0.02≤ΔGFGRDo/f≤0.3  (1-2)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

fLens is the focal length of each lens element in the front-side lens unit,

ΔGFGRDo is an on-axis air space from an image-side surface of the diffractive lens element to an object-side surface in the rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface, and having no other effective transmission surface between the two effective transmission surfaces.

In the image forming lens system of the fourth embodiment, the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface, and the diffractive lens component is disposed closest to the image side in the front-side lens unit, in the same manner as in the image forming lens system of the third embodiment. Thus, the operation effect described in the image forming lens system of the third embodiment can be obtained.

In the image forming lens system of the fourth embodiment, the diffractive lens component includes a diffractive lens element closest to the image side in the same manner as in the image forming lens system of the third embodiment. Thus, the operation effect described in the image forming lens system of the third embodiment can be obtained.

The technical meaning of Conditional Expression (a) is as described above.

The technical meaning of Conditional Expression (1-2) is the same as the technical meaning of Conditional Expression (1).

It is preferable that following Conditional Expression (1-2′), (1-2″), (1-2′″), (1-2″″), (1-2′″″), (1-2″″″), or (1-2′″″″) be satisfied instead of Conditional Expression (1-2): 0.022≤ΔGFGRDo/f≤0.27  (1-2′) 0.025≤ΔGFGRDo/f≤0.25  (1-2″) 0.03≤ΔGFGRDo/f≤0.25  (1-2′″) 0.03≤ΔGFGRDo/f≤0.2  (1-2″″) 0.05≤ΔGFGRDo/f≤0.17  (1-2′″″) 0.06≤ΔGFGRDo/f≤0.2  (1-2″″″) 0.07≤ΔGFGRDo/f≤0.15  (1-2′″″″).

An image forming lens system of a fifth embodiment has the common configuration. In addition, the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface, the diffractive lens component is disposed closest to the image side in the front-side lens unit, the rear-side lens unit includes a positive lens and a negative lens, the air space between the diffractive lens component and the rear-side lens unit is largest of air spaces in the first lens unit, the image forming lens system further includes an aperture stop disposed closer to the object side than the second lens unit to limit on-axis light flux, and following Conditional Expression (1-3) is satisfied: 0.01≤ΔGFDoGRmax/f≤0.2  (1-3)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

ΔGFDoGRmax is a largest on-axis air space of on-axis air spaces from an image-side surface of the diffractive lens component to an object-side surface in the rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface, and having no other effective transmission surface between the two effective transmission surfaces.

In the image forming lens system of the fourth embodiment, the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface, and the diffractive lens component is disposed closest to the image side in the front-side lens unit, in the same manner as in the image forming lens system of the third embodiment. Thus, the operation effect described in the image forming lens system of the third embodiment can be obtained.

The image forming lens system of the second embodiment includes an aperture stop disposed closer to the object side than the second lens unit to limit on-axis light flux.

Since the first lens unit has a positive refractive power, a strong convergence action occurs in the first lens unit. Then, the aperture stop is disposed on the object side of the first lens unit, and the second lens unit is disposed on the image side of the aperture stop. By doing so, it is possible to reduce the diameter of the second lens unit. Furthermore, since the second lens unit is a focus lens unit, it is possible to form a very compact focus lens unit.

The technical meaning of Conditional Expression (1-3) is the same as the technical meaning of Conditional Expression (1).

It is preferable that following Conditional Expression (1-3′), (1-3″), (1-3′″), or (1-3″″) be satisfied instead of Conditional Expression (1-3): 0.015≤ΔGFDoGRmax/f≤0.18  (1-3′) 0.02≤ΔGFDoGRmax/f≤0.17  (1-3″) 0.03≤ΔGFDoGRmax/f≤0.16  (1-3′″) 0.05≤ΔGFDoGRmax/f≤0.15  (1-3″″).

In the image forming lens system of the first embodiment and the image forming lens system of the second embodiment, it is preferable that each lens component in the front-side lens unit be a diffractive lens component having a positive refractive power or a single lens having a positive refractive power.

Thus, it is possible to enhance the positive refractive power in the front-side lens unit and enhance telephoto action.

In the image forming lens system of the first embodiment and the image forming lens system of the second embodiment, it is preferable that the front-side lens unit be composed of all lens elements that are positioned closer to the object side than a predetermined negative lens element, the predetermined negative lens element be a negative lens element disposed closest to the object side, of negative lens elements that satisfy following Conditional Expression (a-1), and the negative lens element of the rear-side lens unit be the predetermined negative lens element: 0.4≤|f/fLn|  (a-1)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and

fLn is a focal length of the predetermined negative lens element.

As described above, the volume of the front-side lens unit is largest in the optical system. Hence, in order to reduce the weight of the optical system, it is preferable that a lens element having a large weight be not disposed in the front-side lens unit.

A negative lens element that satisfies Conditional Expression (a-1) is a lens element having large refractive power, that is, a negative lens element having a large weight. Hence, it is preferable that a negative lens element that satisfies Conditional Expression (a-1) be not included in the front-side lens unit.

In order that a negative lens element that satisfies Conditional Expression (a-1) is not made to be included in the front-side lens unit, it is desirable that the front-side lens unit and the rear-side lens unit are separated with the predetermined negative lens element on the boundary. The predetermined negative lens is a negative lens element located closest to the object side in the rear-side lens unit, among the negative lens elements that satisfy Conditional Expression (a-1).

By making such that the front-side lens unit includes all the lens elements located closer to the object side than the predetermined negative lens element, a lens element having a large weight is not included in the front-side lens unit. When not only a lens element but also a lens component is located closer to the object side than the predetermined negative lens element, the lens component is also included in the front-side lens unit.

The lens element that does not reach the lower limit value of Conditional Expression (a-1) is a lens element having small refractive power. Therefore, even when a negative lens element is disposed in the front-side lens unit, the volume of the lens is not increased as long as the lens element that dose not reach the lower limit value of Conditional Expression (a-1).

Furthermore, because the refractive power is small, even when such a lens element is disposed in the front-side lens unit, it is possible to ensure positive refractive power necessary for the front-side lens unit with no problem. Hence, it is acceptable that a lens that does not reach the lower limit value of Conditional Expression (a-1) is disposed in the front-side lens unit.

In the image forming lens system of the third embodiment and the image forming lens system of the fourth embodiment, it is preferable that the front-side lens unit be composed of all lens elements that are positioned closer to the object side than a predetermined negative lens element, and the predetermined negative lens element be a negative lens element disposed closest to the object side, of negative lens elements that satisfy following Conditional Expression (a-1): 0.4≤|f/fLn|  (a-1)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and

fLn is the focal length of the predetermined negative lens element.

The technical meaning of Conditional Expression (a-1) is as described above.

In the image forming lens system of the third embodiment, it is preferable that each lens component in the front-side lens unit be a diffractive lens component or a single lens having a positive refractive power, and the diffractive lens component have a positive refractive power.

Thus, since a diffractive lens component having a positive refractive power is included, it is possible to enhance the refractive power of the front-side lens unit while correcting chromatic aberration and to enhance telephoto action.

In the image forming lens system of the fifth embodiment, it is preferable that, in the front-side lens unit, a positive lens element be disposed closest to the object side and the diffractive lens component be disposed closest to the image side, each lens component in the front-side lens unit be a diffractive lens component or a single lens having a positive refractive power, and the diffractive lens component have a positive refractive power.

Thus, by setting a diffractive lens component at a position closest to the image side in the front-side lens unit, it is possible to enhance the hood effect at a position closer to the object side than the diffractive lens component and to reduce flare.

In the image forming lens system of the fifth embodiment, it is preferable that the front-side lens unit be composed of all lens elements that are positioned closer to the object side than a predetermined negative lens element, the predetermined negative lens element be a negative lens element disposed closest to the object side, of negative lens elements that satisfy following Conditional Expression (a-1′), and the negative lens element of the rear-side lens unit be the predetermined negative lens element: 0.5≤|f/fLn|  (a-1′)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and

fLn is the focal length of the predetermined negative lens element.

In the image forming lens system of the first embodiment, the image forming lens system of the fourth embodiment, and the image forming lens system of the fifth embodiment, it is preferable that the rear-side lens unit include a first rear-side lens unit and a second rear-side lens unit with an air space interposed therebetween, the first rear-side lens unit includes a negative lens element and a positive lens element, and the second rear-side lens unit includes a positive lens element.

In the image forming lens system of the first embodiment, the image forming lens system of the fourth embodiment, and the image forming lens system of the fifth embodiment, the rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit with an air space interposed therebetween, and the first rear-side lens unit includes a negative lens element and a positive lens element, in the same manner as in the image forming lens system of the second embodiment. Thus, the operation effect described in the image forming lens system of the second embodiment can be obtained.

In the image forming lens system of the first embodiment to the image forming lens system of the fifth embodiment (hereinafter referred to as “the image forming lens system of the present embodiment”), it is preferable that the first rear-side lens unit have a negative refractive power.

By doing so, it is possible to further enhance telephoto action. As a result, it becomes easy to reduce the total length of the optical system. Furthermore, the optical symmetry (the arrangement of positive refractive power, negative refractive power, and positive refractive power) in the first lens unit can be obtained more easily. Therefore, it is possible to further enhance the correction effect for spherical aberration, astigmatism, longitudinal chromatic aberration, and chromatic aberration of magnification in the first lens unit.

In the image forming lens system of the present embodiment, it is preferable that the second rear-side lens unit have a positive refractive power.

By doing so, the optical symmetry (the arrangement of positive refractive power, negative refractive power, and positive refractive power) can be obtained more easily in the first lens unit. Therefore, it is possible to further enhance the correction effect for spherical aberration, astigmatism, longitudinal chromatic aberration, and chromatic aberration of magnification in the first lens unit.

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (1) be satisfied: 0.015≤ΔGFGR/f≤0.25  (1)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and

ΔGFGR is the on-axis air space from an image-side surface in the front-side lens unit to an object-side surface in the rear-side lens unit.

The technical meaning of Conditional Expression (1) is as described above. Moreover, following Conditional Expression (1′″″) may be satisfied: 0.015≤ΔGFGR/f≤0.30  (1′″″).

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (2) be satisfied: 0.1≤ΔGFFGR1/f≤0.5  (2)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and

ΔGFFGR1 is the on-axis distance from an object-side surface in the front-side lens unit to an object-side surface in the first rear-side lens unit.

The technical meaning of Conditional Expression (2) is as described above.

In the image forming lens system of the present embodiment, it is preferable that the diffractive lens component be disposed closest to the image side in the front-side lens unit and include a diffractive lens element closest to the image side, and following Conditional Expression (2-1) be satisfied: 0.10≤ΔGFFGR1Do/f≤0.5  (2-1)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance,

ΔGFFGR1Do is an on-axis distance from an object-side surface of the diffractive lens element to an object-side surface in the first rear-side lens unit,

a lens component means a lens in which only two surfaces which are an object-side surface and an image-side surface are effective surfaces in contact with air, and

a lens element is a lens having two effective transmission surfaces which are an object-side surface and an image-side surface and having no other effective transmission surface between the two effective transmission surfaces.

The technical meaning of Conditional Expression (2-1) is the same as the technical meaning of Conditional Expression (2).

It is preferable that following Conditional Expression (2-1′) or (2-1″) be satisfied instead of Conditional Expression (2-1): 0.12≤ΔGFFGR1Do/f≤0.45  (2-1′) 0.13≤ΔGFFGR1Do/f≤0.4  (2-1″).

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (3) be satisfied: 50≤νdGFave  (3)

where

νdGFave is an average Abbe number of positive lens elements in the front-side lens unit.

The value of the average Abbe number in Conditional Expression (3) can be obtained by averaging the Abbe numbers of the positive lens elements included in the front-side lens unit.

In an image forming lens system, especially in a telephoto lens, size reduction of the optical system is required in addition to ensuring favorable imaging performance. In order to obtain favorable imaging performance in a telephoto lens, it is mainly necessary to suppress occurrence of longitudinal chromatic aberration. On the other hand, in order to achieve size reduction of the optical system, it is necessary to increase the positive refractive power of a lens unit positioned closer to the object side. However, in doing so, longitudinal chromatic aberration is more likely to occur.

In a case of falling below a lower limit value of Conditional Expression (3), since the amount of occurrence of longitudinal chromatic aberration is increased, it becomes difficult to reduce the overall length of the optical system.

It is preferable that following Conditional Expression (3′), (3″), or (3′″) be satisfied instead of Conditional Expression (3): 60≤νdGFave  (3′) 70≤νdGFave  (3″) 77≤νdGFave  (3′″).

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (4) be satisfied: 50≤νdGFmax  (4)

where

νdGFmax is a largest Abbe number of Abbe numbers of positive lens elements in the front-side lens unit.

The technical meaning of Conditional Expression (4) is the same as the technical meaning of Conditional Expression (3).

It is preferable that following Conditional Expression (4′) or (4″) be satisfied instead of Conditional Expression (4): 60≤νdGFmax  (4′) 70≤νdGFmax  (4″).

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (5) be satisfied: 0.008≤DGR1GR2/f≤0.3  (5)

where,

DGR1GR2 is an axial air space between the first rear-side lens unit and the second rear-side lens unit; and

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance.

In a case of falling below a lower limit value of Conditional Expression (5), the difference is reduced between the light ray height in the first rear-side lens unit and the light ray height in the second rear-side lens unit. Therefore, it becomes impossible to enhance the correction effect for aberrations such as spherical aberration, astigmatism, longitudinal chromatic aberration, and chromatic aberration of magnification. In a case of exceeding an upper limit value of Conditional Expression (5), it becomes difficult to reduce the overall length of the optical system.

It is preferable that following Conditional Expression (5′), (5″), (5′″), (5″″), or (5′″″) be satisfied instead of Conditional Expression (5): 0.01≤DGR1GR2/f≤0.28  (5′) 0.015≤DGR1GR2/f≤0.27  (5″) 0.02≤DGR1GR2/f≤0.25  (5′″) 0.025≤DGR1GR2/f≤0.23  (5″″) 0.03≤DGR1GR2/f≤0.2  (5′″″).

In the image forming lens system of the present embodiment, it is preferable that the front-side lens unit have a plurality of positive lens elements.

By doing so, it is possible to distribute positive refractive power among a plurality of lenses even when the refractive power of the front-side lens unit is increased. Therefore, it is possible to suppress occurrence of spherical aberration. Moreover, since it is possible to enhance telephoto action in the first lens unit, it is easy to reduce the overall length of the optical system.

In the image forming lens system lens of the present embodiment, it is preferable that following Conditional Expression (6) be satisfied: 0.2≤fGF/f≤0.8  (6)

where,

fGF is a focal length of the front-side lens unit; and

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance.

In a case of falling below a lower limit value of Conditional Expression (6), the amount of occurrence of axial chromatic aberration and the amount of occurrence of spherical aberration in the front-side lens unit is increased. Therefore, favorable imaging performance cannot be obtained. In a case of exceeding an upper limit value of Conditional Expression (6), it becomes difficult to reduce the size of the optical system.

It is preferable that following Conditional Expression (6′) or (6″) be satisfied instead of Conditional Expression (6): 0.25≤fGF/f≤0.6  (6′) 0.28≤fGF/f≤0.47  (6″).

In the image forming lens system of the present embodiment, it is preferable that a first lens element is disposed closest to the object and following Conditional Expression (7) be satisfied: 1.6≤fL1/fGF≤5.0  (7)

where,

fL1 is a focal length of the first lens element; and

fGF is the focal length of the front-side lens unit.

In a case of falling below a lower limit value of Conditional Expression (7), the positive refractive power of the first lens element becomes too large. In this case, the amount of occurrence of spherical aberration in the front-side lens unit is increased. Therefore, it becomes difficult to reduce the size of the optical system.

In a case of exceeding an upper limit value of Conditional Expression (7), the positive refractive power of the first lens element becomes too small. In this case, the positive refractive power of the front-side lens unit becomes also small. If an adequate positive refractive power is to be ensured in the front-side lens unit, the load of refractive power in the lenses positioned closer to the image side than the first lens element is increased. As a result, the amount of occurrence of spherical aberration in the front-side lens unit is increased. Therefore, it becomes difficult to reduce the size of the optical system.

It is preferable that following Conditional Expression (7′), (7″), or (7″″) be satisfied instead of Conditional Expression (7): 1.8≤fL1/fGF≤4.5  (7′) 1.9≤fL1/fGF≤4.0  (7″) 2.0≤fL1/fGF≤3.5  (7″).

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (8) be satisfied: −3.0≤fGF/fGR1≤0.1  (8)

where,

fGF is the focal length of the front-side lens unit; and

fGR1 is a focal length of the first rear-side lens unit.

In a case of falling below a lower limit value of Conditional Expression (8), spherical aberration is overcorrected. Therefore, favorable imaging performance cannot be obtained. In a case of exceeding an upper limit value of Conditional Expression (8), the negative refractive power of the first rear-side lens unit becomes too small. Therefore, it becomes difficult to reduce the overall length of the optical system.

It is preferable that following Conditional Expression (8′), (8″), or (8″″) be satisfied instead of Conditional Expression (8): −2.5≤fGF/fGR1≤0.0  (8′) −2.0≤fGF/fGR1≤0.2  (8″) −1.8≤fGF/fGR1≤−0.3  (8″″).

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (9) be satisfied: 0.06≤|fG2/f|≤0.25  (9)

where,

fG2 is a focal length of the second lens unit; and

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance.

In the second lens unit, light rays are refracted in a direction away from the optical axis. In a case of falling below a lower limit value of Conditional Expression (9), light rays are refracted in a direction further away from the optical axis. As a result, the lens diameter of the third lens unit is increased. Therefore, it becomes difficult to reduce the size of the optical system.

The second lens unit functions as a focus lens unit. In a case of exceeding an upper limit value of Conditional Expression (9), the refractive power of the second lens unit becomes too small. In this case, since the amount of movement of the imaging position relative to the amount of movement of the second lens unit (focus lens unit) (hereinafter referred to as “focus sensitivity”) is too small, the amount of movement of the second lens unit at the time of focusing is increased. Therefore, it becomes difficult to reduce the overall length of the optical system. Moreover, telephoto action obtained with the first lens unit and the second lens unit is reduced. Therefore, it becomes difficult to reduce the overall length of the optical system.

It is preferable that following Conditional Expression (9′) or (9″) be satisfied instead of Conditional Expression (9): 0.07≤|fG2/f|≤0.2  (9′) 0.08≤|fG2/f|≤0.15  (9″).

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (10) be satisfied: 3.0≤|MGG2B ²×(MGG2−1)|≤6.5  (10),

where,

MGG2B is a lateral magnification of a first rear-side lens system;

MGG2 is a lateral magnification of the second lens unit;

the lateral magnification is a lateral magnification at the time of focusing on an infinite object; and

the first rear-side lens system is a lens system constituted with all of the lenses positioned closer to the image side than the second lens unit.

In a case of falling below a lower limit value of Conditional Expression (10), the amount of movement of the second lens unit at the time of focusing is too large. Therefore, it becomes difficult to reduce the overall length of the optical system. In a case of exceeding an upper limit value of Conditional Expression (10), it becomes difficult to control the position of the second lens unit at the time of focusing. Therefore, accurate focusing cannot be achieved.

It is preferable that following Conditional Expression (10′), (10″), (10′″), or (10″″) be satisfied instead of Conditional Expression (10): 3.2≤|MGG2B ²×(MGG2²−1)|≤6.3  (10′) 3.4≤|MGG2B ²×(MGG2²−1)|≤6.1  (10″) 3.6≤|MGG2B ²×(MGG2²−1)|≤6.0  (10′″) 4.0≤|MGG2B ²×(MGG2²−1)|≤5.6  (10″″).

In the image forming lens system of the present embodiment, it is preferable that the second lens unit include two or less lens elements.

As described above, in the image forming lens system of the present embodiment, the optical symmetry is ensured. Thus, coma, distortion, and chromatic aberration of magnification are corrected favorably. Therefore, even when the arrangement of the second lens unit is simplified, it is possible to ensure high imaging performance at the time of focusing.

Then, the second lens unit is constituted with two or less lenses. By doing so, it is possible to reduce the weight of the second lens unit, that is, the focus lens unit, while keeping high imaging performance even at the time of focusing.

In the image forming lens system of the present embodiment, it is preferable that the second lens unit include a negative lens element and a positive lens element.

To reduce the weight of the second lens unit, it is desirable that the second lens unit is configured two or less lens elements. The image forming lens system of the present embodiment has the common configuration. Hence, it is possible to reduce the diameter of the second lens unit.

Even when a positive lens element and a negative lens element are used as two lens elements, it is possible to reduce the weight of the second lens unit. In this case, since the second lens unit has a positive lens element and a negative lens element, it is possible to further reduce variation of chromatic aberration at a time of focusing.

In the image forming lens system of the present embodiment, it is preferable that the second lens unit include one negative lens element and one positive lens element.

By doing so, it is possible to reduce the amount of occurrence of longitudinal chromatic aberration and the amount of occurrence of chromatic aberration of magnification in the second lens unit. As a result, it is possible to ensure stable imaging performance at the time of focusing. Furthermore, the second lens unit includes two lens elements whereby it is possible to reduce the weight of the second lens unit while keeping high imaging performance.

In the image forming lens system of the present embodiment, it is preferable that the third lens unit have a camera shake-correction lens unit and the camera shake-correction lens unit move in a direction vertical to the optical axis.

By moving one or more lenses in a direction vertical to the optical axis, it is possible to correct a shift of the imaging position due to camera shake. Here, if the lens to be moved (hereinafter referred to as “camera shake-correction lens”) is compact and lightweight, it is possible to quickly correct a shift of the imaging position.

In a telephoto-type optical system, the second lens unit and the third lens unit are units having a small diameter. Then, by constituting the camera shake-correction lens unit with the lenses in the third lens unit, it is possible to reduce the diameter and weight of the camera shake-correction lens. Thus, it is possible to enhance the responsivity of the camera shake-correction lens unit. As a result, it is possible to correct a shift of the imaging position due to camera shake at high speed.

In the image forming lens system of the present embodiment, it is preferable that the third lens unit have a sub-lens unit on the object side and the sub-lens unit have a refractive power with a sign different from that of the camera shake-correction lens unit.

By doing so, it is possible to increase the amount of shift of the imaging position relative to the amount of shift of the camera shake-correction lens unit (hereinafter referred to as “camera shake-correction sensitivity”). That is, it is possible to reduce the amount of shift of the camera shake-correction lens unit. As a result, it is possible to correct a shift of the imaging position due to camera shake at high speed.

In the image forming lens system of the present embodiment, it is preferable that the third lens unit have a sub-lens unit on the image side and the sub-lens unit have a refractive power with a sign different from that of the camera shake-correction lens unit.

By doing so, it is possible to increase the camera shake-correction sensitivity. That is, it is possible to reduce the amount of shift of the camera shake-correction lens unit. As a result, it is possible to correct a shift of the imaging position due to camera shake at high speed.

In the image forming lens system of the present embodiment, it is preferable that the third lens unit have an object-side sub-lens unit and an image-side sub-lens unit on the object side and the image side, respectively, and the object-side sub-lens unit and the image-side sub-lens unit both have a refractive power with a sign different from that of the camera shake-correction lens unit.

By doing so, it is possible to increase the camera shake-correction sensitivity. That is, it is possible to reduce the amount of shift of the camera shake-correction lens unit. As a result, it is possible to correct a shift of the imaging position due to camera shake at high speed.

In the image forming lens system of the present embodiment, it is preferable that the camera shake-correction lens unit at least have a first correction lens element, a second correction lens element, and a third correction lens element, and the first correction lens element and the second correction lens element have a refractive power with the same sign as the camera shake-correction lens unit, and the third correction lens element have a refractive power with a sign different from that of the camera shake-correction lens unit.

The camera shake-correction lens unit moves in a direction vertical to the optical axis. This movement mainly causes variation of spherical aberration, astigmatism, and chromatic aberration of magnification. If the amount of this variation is large, the imaging performance is reduced.

Then, in the image forming lens system of the present embodiment, a refractive power with the same sign as the camera shake-correction lens unit is imparted to the first correction lens element and the second correction lens element. By doing so, the refractive power of the camera shake-correction lens unit is distributed between the first correction lens element and the second correction lens element. As a result, the refractive power of the first correction lens element and the refractive power of the second correction lens element are both reduced. Therefore, it is possible to reduce the amount of variation of spherical aberration and the amount of variation of astigmatism.

Furthermore, a refractive power with a sign different from that of the camera shake-correction lens unit is imparted to the third correction lens element. By doing so, it is possible to reduce the amount of variation of chromatic aberration of magnification.

In the image forming lens system of the present embodiment, it is preferable that the camera shake-correction lens unit have a negative refractive power.

The camera shake-correction lens unit moves in a direction vertical to the optical axis. Therefore, the camera shake-correction lens unit preferably has a small diameter. By setting the lens unit positioned where light rays converge more, as the camera shake-correction lens unit, it is possible to reduce the size of the camera shake-correction lens unit. Then, by setting the refractive power of the camera shake-correction lens unit to a negative refractive power, it is possible to further reduce the size of the camera shake-correction lens unit.

In the image forming lens system of the present embodiment, it is preferable that the third lens unit have an object-side sub-lens unit having a positive refractive power, a camera shake-correction lens unit having a negative refractive power, and an image-side sub-lens unit having a positive refractive power.

As described above, in image forming lens system of the present embodiment, the first lens unit has a positive refractive power and the second lens unit has a negative refractive power. Therefore, since telephoto action can be obtained, it is possible to reduce the overall length of the optical system.

Since the first lens unit has a positive refractive power, light rays converge on the side closer to the image side than the first lens unit. That is, the height of light rays is low at the position of the second lens unit. Therefore, the outer diameter of the second lens unit is reduced. Then, the second lens unit is set as the focus lens unit and focusing is performed in the second lens unit. By doing so, it is possible to reduce the outer diameter of the focus lens unit.

In the focus lens unit, when the refractive power is increased, the focus sensitivity becomes high. When the focus sensitivity is high, the amount of movement of the focus lens unit at the time of focusing is reduced. As described above, the second lens unit functions as the focus lens unit. Then, the refractive power of the second lens unit is increased. By doing so, it is possible to enhance the focus sensitivity. As a result, it is possible to reduce the amount of movement of the focus lens unit at the time of focusing.

A focus unit has a focus lens unit and a focus mechanism. Achieving a smaller diameter of the focus lens unit and reducing the amount of movement at the time of focusing can also reduce the size and weight of the entire focus unit.

Furthermore, light incident on the focus lens unit is converging light. Therefore, even when the refractive power of the focus lens unit is increased, it is possible to reduce divergence of light rays passing through the focus lens unit. As a result, it is possible to reduce the diameter of the entire lens system positioned closer to the image side than the second lens unit while enhancing focus sensitivity.

By arranging a positive lens unit on the image side of the second lens unit, it is possible to easily enhance the focus sensitivity.

The camera shake-correction lens unit moves in a direction vertical to the optical axis. It is preferable to minimize the range of movement of the camera shake-correction lens unit. Based on this, it is desirable that a lens unit positioned where the height of light rays is low be set as the camera shake-correction lens unit.

As described above, on the side closer to the image side than the first lens unit, the height of light rays is low. Thus, it is preferable that the camera shake-correction lens unit be provided in the second lens unit or the third lens unit. However, the second lens unit functions as a focus lens unit. Based on this, it is preferable that the camera shake-correction lens unit be disposed in the third lens unit.

Here, the third lens unit is constituted with an object-side sub-lens unit having a positive refractive power, a camera shake-correction lens unit having a negative refractive power, and an image-side sub-lens unit having a positive refractive power.

In such an arrangement, the lens units having a positive refractive power are disposed on both sides of the camera shake-correction lens unit. Therefore, it is possible to increase the camera shake-correction sensitivity. That is, it is possible to reduce the amount of shift of the camera shake-correction lens unit. As a result, it is possible to correct a shift of the imaging position due to camera shake at high speed.

As described above, on the side closer to the image side than the first lens unit, the height of light rays is low. Then, an arrangement may be such that the second lens unit has a camera shake-correcting function and the third lens unit has the focusing function. However, in such an arrangement, variation of comatic aberration due to camera shake correction is increased by movement of the lens unit at the time of focusing. Thus, such an arrangement is not preferable.

When the focus lens unit has a negative refractive power, it is possible to enhance focus sensitivity by disposing a positive lens unit on the image side of the focus lens unit. Furthermore, when the camera shake-correction lens unit has a negative refractive power, it is possible to enhance camera shake-correction sensitivity by disposing a positive lens unit on the object side of the camera shake-correction lens unit.

The object-side sub-lens unit is positioned between the second lens unit and the camera shake-correction lens unit. The second lens unit has a negative refractive power and functions as a focus lens unit. Then, when the refractive power of the object-side sub-lens unit is set as a positive refractive power, it follows that a positive lens unit is positioned on the image side of the focus lens unit, and therefore it is possible to enhance the focus sensitivity.

Furthermore, the object-side sub-lens unit is positioned on the object side of the camera shake-correction lens unit. It follows that a positive lens unit is positioned on the object side of the camera shake-correction lens unit, and therefore it is possible to enhance the camera shake-correction sensitivity. In this way, by setting the refractive power of the object-side sub-lens unit as a positive refractive power, it is possible to enhance focus sensitivity and camera shake-correction sensitivity at the same time.

It is preferable that the second lens unit and the third lens unit be disposed closer to the image side than the aperture stop. By doing so, it is possible to further reduce the diameters of the focus lens unit and the camera shake-correction lens unit.

At the time of focusing, a lens unit moves along the optical axis. When the lens unit moves, imaging performance tends to be degraded mainly due to variation of spherical aberration and variation of longitudinal chromatic aberration. In order to alleviate degradation of imaging performance, it is necessary to alleviate variation of spherical aberration and variation of longitudinal chromatic aberration. It is then desirable that the second lens unit at least have a positive lens and a negative lens. By doing so, it is possible to prevent degradation of imaging performance at the time of focusing.

Variation of spherical aberration and variation of longitudinal chromatic aberration in the second lens unit change through the object-side sub-lens unit. Therefore, it is desirable that the object-side sub-lens unit have a positive lens and a negative lens. By doing so, it is possible to reduce variation of spherical aberration and variation of longitudinal chromatic aberration.

At the time of camera shake correction, the lens unit moves in a direction vertical to the optical axis. When the lens unit moves, imaging performance tends to be degraded mainly due to variation of spherical aberration, variation of curvature of field, and variation of chromatic aberration of magnification. In order to alleviate degradation of imaging performance, it is necessary to alleviate variation of spherical aberration, variation of curvature of field, and variation of chromatic aberration of magnification. It is then desirable that the camera shake-correction lens unit at least have one positive lens and two negative lenses. By providing two negative lenses, it is possible to distribute the refractive power of the camera shake-correction lens unit between the two negative lenses. As a result, it is possible to prevent degradation of imaging performance at the time of camera shake correction.

It is desirable that the second lens unit include two lenses, that the object-side sub-lens unit include two or less lenses, and that the camera shake-correction lens unit include three lenses. By doing so, it is possible to construct an optical system having high focusing performance and high camera shake-correcting function with a smaller number of lenses.

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (13) be satisfied: 1.0<|MGISB×(MGIS−1)|<4.0  (13)

where,

MGISB is a lateral magnification of a second rear-side lens system;

MGIS is a lateral magnification of the camera shake-correction lens unit;

the lateral magnification is the lateral magnification at the time of focusing on an infinite object; and

the second rear-side lens system is a lens system constituted with all of the lenses positioned closer to the image side than the camera shake-correction lens unit.

In a case of falling below a lower limit value of Conditional Expression (13), a sufficient camera shake-correction effect cannot be obtained. In a case of exceeding an upper limit value of Conditional Expression (13), the refractive power of the camera shake-correction lens unit is increased. In this case, variation of spherical aberration, variation of curvature of field, and variation of chromatic aberration of magnification are increased. Therefore, it becomes difficult to correct spherical aberration, astigmatism, and chromatic aberration of magnification.

It is preferable that following Conditional Expression (13′) or (13″) be satisfied instead of Conditional Expression (13): 1.3<|MGISB×(MGIS−1)|<3.0  (13′) 1.5<|MGISB×(MGIS−1)|<2.7  (13″).

In the image forming lens system of the present embodiment, it is preferable that the aperture stop be disposed on the object side of the second lens unit.

Since the first lens unit has a positive refractive power, a strong convergence action occurs in the first lens unit. Then, the aperture stop is disposed on the object side of the first lens unit, and the second lens unit is disposed on the image side of the aperture stop. By doing so, it is possible to reduce the diameter of the second lens unit. Furthermore, since the second lens unit is a focus lens unit, it is possible to form a very compact focus lens unit.

In the image forming lens system of the present embodiment, it is preferable that the aperture stop be disposed between the first lens unit and the second lens unit.

Since the first lens unit has a positive refractive power, a strong convergence action occurs in the first lens unit. Then, the aperture stop is disposed on the object side of the first lens unit, and the second lens unit is disposed on the image side of the aperture stop. By doing so, it is possible to reduce the diameter of the second lens unit. Furthermore, since the second lens unit is a focus lens unit, it is possible to form a very compact focus lens unit.

Furthermore, the first lens unit includes the front-side lens unit and the first rear-side lens unit from the object side. The front-side lens unit has a positive refractive power, and the first rear-side lens unit has a negative refractive power. Then, the second lens unit and the third lens unit are disposed on the image side of the first lens unit. The second lens unit has a negative refractive power, and the third lens unit has a positive refractive power.

When the aperture stop is made to be disposed between the first lens unit and the second lens unit, the arrangement of refractive power is symmetry between the object side and the image side of the aperture stop. Therefore, it is possible to favorably perform correction of off-axis aberration, mainly correction of chromatic aberration of magnification and distortion.

In the image forming lens system of the present embodiment, it is preferable that the aperture stop be disposed between the first rear-side lens unit and the second lens unit.

Furthermore, the first lens unit includes the front-side lens unit and the first rear-side lens unit from the object side. The front-side lens unit has a positive refractive power, and the first rear-side lens unit has a negative refractive power. Then, the second lens unit and the third lens unit are disposed on the image side of the first lens unit. The second lens unit has a negative refractive power, and the third lens unit has a positive refractive power.

When the aperture stop is made to be disposed between the first rear-side lens unit and the second lens unit, the arrangement of refractive power is symmetry between the object side and the image side of the aperture stop. Therefore, it is possible to favorably perform correction of off-axis aberration, mainly correction of chromatic aberration of magnification and distortion.

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (14) be satisfied: 0.19≤DGFairmax/DGF≤1.0  (14)

where,

DGFairmax is a largest axial air space, of axial air spaces in the front-side lens unit; and

DGF is an axial thickness of the front-side lens unit.

In a case of falling below a lower limit value of Conditional Expression (14), since the weight of the positive lenses in the front-side lens unit is increased, it becomes difficult to make the optical system light weight. In a case of exceeding an upper limit value of Conditional Expression (14), it becomes difficult to reduce the overall length of the optical system.

It is preferable that following Conditional Expression (14′) or (14″) be satisfied instead of Conditional Expression (14): 0.2≤DGFairmax/DGF≤0.9  (14′) 0.25≤DGFairmax/DGF≤0.8  (14″).

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (1-4) be satisfied: 0.01≤ΔGFGRmax/f≤0.2  (1-4)

where

f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and

ΔGFGRmax is a largest on-axis air space of on-axis air spaces from an image-side surface in the front-side lens unit to an object-side surface in the rear-side lens unit.

The technical meaning of Conditional Expression (1-4) is the same as the technical meaning of Conditional Expression (1).

It is preferable that following Conditional Expression (1-4′), (1-4″), (1-4′″), or (1-4″″) be satisfied instead of Conditional Expression (1-4): 0.015≤ΔGFGRmax/f≤0.18  (1-4′) 0.02≤ΔGFGRmax/f≤0.17  (1-4″) 0.03≤ΔGFGRmax/f≤0.16  (1-4′″) 0.05≤ΔGFGRmax/f≤0.15  (1-4″″).

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (15) be satisfied: 0.05≤ΔGFGR/fGF≤0.4  (15)

where

ΔGFGR is the on-axis air space from the image-side surface in the front-side lens unit to the object-side surface in the rear-side lens unit, and

fGF is the focal length of the front-side lens unit.

In a case of falling below a lower limit value of Conditional Expression (15), the light flux-converging action in the front-side lens unit becomes weak. In this case, the outer diameter in the rear-side lens unit is increased. Therefore, it becomes difficult to reduce the size and weight of the first lens unit.

In a case of exceeding an upper limit value of Conditional Expression (15), the amount of occurrence of spherical aberration in the front-side lens unit is increased. Therefore, it becomes difficult to correct spherical aberration favorably with the lenses positioned closer to the image side than the front-side lens unit.

It is preferable that following Conditional Expression (15′) or (15″) be satisfied instead of Conditional Expression (15): 0.06≤ΔGFGR/fGF≤0.35  (15′) 0.08≤ΔGFGR/fGF≤0.33  (15″).

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (16) be satisfied: 50≤νdLp1  (16)

where,

νdLp1 is an Abbe number of a positive lens positioned closest to the object.

In a case of falling below a lower limit value of Conditional Expression (16), since longitudinal chromatic aberration in the first sub-lens unit is increased, favorable imaging performance is not achieved. Alternatively, when favorable imaging performance is to be achieved, it is difficult to reduce the overall length of the optical system.

It is preferable that any one of following Conditional Expressions (16′), (16″), or (16′″) be satisfied instead of Conditional Expression (16): 55≤νdLp1  (16′) 62≤νdLp1  (16″) 65≤νdLp1  (16′″).

In the image forming lens system of the present embodiment, it is preferable that the front-side lens unit include two positive lenses.

By doing so, it is becomes easy to reduce the amount of occurrence of spherical aberration in the front-side lens unit, to reduce the weight of the optical system, and to reduce the overall length of the optical system.

In the image forming lens system of the present embodiment, it is preferable that the first rear-side lens unit have at least two negative lenses.

In the front-side lens unit, a negative lens having a large refractive power is not disposed. Therefore, it is difficult to correct chromatic aberration favorably in the front-side lens unit. Accordingly, in the first rear-side lens unit, it is necessary to enhance the function of correcting spherical aberration and longitudinal chromatic aberration remaining in the front-side lens unit. Then, by using at least two negative lenses in the first rear-side lens unit, it is possible to correct both spherical aberration and longitudinal chromatic aberration favorably while reducing the overall length of the optical system.

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (17) be satisfied: 1.5≤|fG1/fG2|≤6.5  (17)

where,

fG1 is a focal length of the first lens unit; and

fG2 is the focal length of the second lens unit.

In a case of falling below a lower limit value of Conditional Expression (17), the amount of occurrence of spherical aberration in the first lens unit is increased. Therefore, favorable imaging performance cannot be obtained. In a case of exceeding an upper limit value of Conditional Expression (17), focus sensitivity is reduced. In this case, the amount of movement of the second lens unit at the time of focusing is increased. Therefore, the overall length of the optical system is increased.

It is preferable that following Conditional Expression (17′) or (17″) be satisfied instead of Conditional Expression (17): 2.0≤|fG1/fG2|≤6.0  (17′) 2.2≤|fG1/fG2|≤5.0  (17″).

In the image forming lens system of the present embodiment, it is preferable that a lens unit that moves in the optical axis direction be the second lens unit alone.

It is possible to further reduce a change in centroid due to the movement of the lens units.

In the image forming lens system of the present embodiment, it is preferable that the third lens unit include a positive lens element and a negative lens element.

By doing so, it is possible to perform chromatic aberration correction in the third lens unit more favorably.

In the image forming lens system of the present embodiment, it is preferable that the front-side lens unit include at least one of a lens element and a lens component, the lens element and the lens component having a positive refractive power.

By doing so, it is possible to adopt a configuration in which a negative lens element having a large weight, that is, a negative lens element having large refractive power is not disposed in the front-side lens unit with the largest volume. As a result, it is possible to significantly reduce the weight of the optical system.

In the image forming lens system of the present embodiment, it is preferable that following Conditional Expression (18) be satisfied: 0.15≤ΣdGF/fGF≤0.7  (18)

where

ΣdGF is a total thickness of the front-side lens unit, and

fGF is the focal length of the front-side lens unit.

In a case of falling below a lower limit value of Conditional Expression (18), number of the lens elements with a large aperture increase. Therefore, it becomes difficult to reduce the weight of the optical system. In a case of exceeding an upper limit value of Conditional Expression (18), it is difficult to reduce the total length of the optical system.

It is preferable that following Conditional Expression (18′) or (18″) be satisfied instead of Conditional Expression (18): 0.17≤ΣdGF/fGF≤0.6  (18′) 0.2≤ΣdGF/fGF≤0.5  (18″).

In the image forming lens system of the present embodiment, it is preferable that the front-side lens unit include a positive lens component disposed closest to the object side and the diffractive lens component, and the diffractive lens component be disposed closest to the image side in the front-side lens unit and include a diffractive lens element closest to the image side.

By doing this, the operation effect described in the image forming lens system of the third embodiment can be obtained.

In the image forming lens system of the present embodiment, it is preferable that the distance between the front-side lens unit and the first rear-side lens unit and the distance between the first rear-side lens unit and the second rear-side lens unit be always constant.

It is possible to reduce centroid position change, and more stable shooting is possible.

In the image forming lens system of the present embodiment, it is preferable that the front-side lens unit include a lens element located closer to the object side than a predetermined negative lens element that satisfies following Conditional Expression (b): 0.02≤DNx/φenp  (b)

DNx is a thickness on an optical axis of the predetermined negative lens element, and

φenp is a maximum diameter of an entrance pupil of the image forming lens system in a state of achieving a longest focal length at a time of focusing to a furthest distance.

The lens element that satisfies Conditional Expression (b) is a lens element with a large thickness. If a lens element with a large thickness is disposed in the front-side lens unit, the size of the front-side lens unit is increased. Thus, it is preferable that the front-side lens unit do not include a negative lens element that satisfies Conditional Expression (b).

The lens element that does not reach the lower limit value of Conditional Expression (b) is a lens element with a small thickness. Therefore, even when a negative lens element is disposed in the front-side lens unit, the volume of the front-side lens unit is not increased as long as the lens element that dose not reach the lower limit value of Conditional Expression (b). Hence, it is acceptable that a lens element that does not reach the lower limit value of Conditional Expression (b) is disposed in the front-side lens unit.

It is preferable that following Conditional Expression (b′), (b″), or (b′″) be satisfied instead of Conditional Expression (b): 0.017≤DNx/φenp  (b′) 0.016≤DNx/φenp  (b″) 0.015≤DNx/φenp  (b′″).

In the image forming lens system of the present embodiment, it is preferable that the total length of the optical system be fixed in all states.

By fixing the total length of the optical system in all states, it is possible to fix the front-side lens unit. The weight of the front-side lens unit tends to be large. By fixing the front-side lens, it is possible to reduce the weight of the front-side lens unit and to reduce a change in centroid of the total system due to the focusing operation. Therefore, more stable shooting becomes possible in all states.

In the image forming lens system of the present embodiment, it is preferable that the lens unit located closest to the image side be fixed in all states.

The lens unit located closest to the image side is a lens unit closest to the image pickup surface. Hence, by fixing the lens unit located closest to the image side in all states, it is possible to alleviate occurrence of dust due to driving of the lens unit and, in addition, it is possible to alleviate adherence of dust to the image pickup surface.

In the image forming lens system of the present embodiment, it is preferable that the aperture stop be fixed in position in all states.

By doing so, the position of the aperture stop becomes stable, and it is possible to reduce errors of the F-number. Furthermore, since variations in incident light flux diameter due to movement of the position of the aperture stop are reduced and incidence of unnecessary light flux can be reduced, it is possible to alleviate occurrence of flare.

In the image forming lens system of the present embodiment, it is preferable that the first lens unit be fixed in all states.

The weight of the first lens unit tends to be large. By fixing the first lens unit in all states, it is possible to reduce change in centroid position of the optical system. As a result, more stable shooting becomes possible in all states.

In the image forming lens system of the present embodiment, it is preferable that the front-side lens unit include a lens located closer to the object side than a predetermined negative lens that satisfies following Conditional Expression (c): 0.007≤DNx/LTLmin  (c)

where

DNx is the thickness on an optical axis of the predetermined negative lens element, and

LTLmin is a minimum total length of the image forming lens system.

The total length of the image forming lens system is the distance from the plane located closest to the object side to the imaging plane.

The lens element that satisfies Conditional Expression (c) is a lens element with a large thickness. If a lens element with a large thickness is disposed in the front-side lens unit, the size of the front-side lens unit is increased. Thus, it is preferable that the front-side lens unit do not include a negative lens element that satisfies Conditional Expression (c).

The lens element that does not reach the lower limit value of Conditional Expression (c) is a lens element with a small thickness. Therefore, even when a negative lens element is disposed in the front-side lens unit, the volume of the front-side lens unit is not increased as long as the lens element that does not reach the lower limit value of Conditional Expression (c). Hence, it is acceptable that the lens element that does not reach the lower limit value of Conditional Expression (c) is disposed in the front-side lens unit.

It is preferable that following Conditional Expression (c′) or (c″) be satisfied instead of Conditional Expression (c): 0.0065≤DNx/LTLmin  (c′) 0.006≤DNx/LTLmin  (c″).

An image pickup apparatus of the present embodiment includes an optical system and an image pickup element having an image plane and converting an image formed on the image plane by the optical system to an electrical signal, and the optical system is the image forming lens system of the present embodiment.

It is possible to provide an image pickup apparatus having excellent portability and producing a high-quality image.

Examples of image forming lens system will be described in details below based on the drawings. It is noted that the present invention is not limited by those Examples.

In the diffractive optical element used in the image forming lens system of Examples below, optical materials different from each other are laminated, and a relief pattern is formed on the boundary surface therebetween. By providing such a structure, a diffractive optical element having high diffraction efficiency in a wide wavelength band is implemented.

The diffractive optical element may be formed of a single optical material. In this case, a relief pattern is formed on a surface of the optical material. The diffractive optical element may be formed of three or more optical materials. In this case, a relief pattern is formed on at least one boundary surface. Of course, a relief pattern may be formed on the air contact surface.

The diffractive optical element for use in the image forming lens system of the present example is not limited to the one having the structure described above. The diffractive optical element for use in the image forming lens system of the present example may be the diffractive optical elements, for example, as described in Japanese Patent Application Laid-open No. 2010-39456, Japanese Patent Application Laid-open No. 2003-215457, and Japanese Patent Application Laid-open No. H11-133305.

The lens cross-sectional diagrams of Examples will be described.

FIG. 1A, FIG. 2A, FIG. 3A, FIG. 4A, FIG. 5A, and FIG. 6A show lens cross-sectional diagrams at a time of focusing to an object point at infinity.

FIG. 1B, FIG. 2B, FIG. 3B, FIG. 4B, FIG. 5B, and FIG. 6B show lens cross-sectional diagrams at a time of focusing to an object point at a close distance.

The aberration diagrams of Examples will be described.

FIG. 7A, FIG. 8A, FIG. 9A, FIG. 10A, FIG. 11A, and FIG. 12A show spherical aberration (SA) at a time of focusing to an object point at infinity.

FIG. 7B, FIG. 8B, FIG. 9B, FIG. 10B, FIG. 11B, and FIG. 12B show astigmatism (AS) at a time of focusing to an object point at infinity.

FIG. 7C, FIG. 8C, FIG. 9C, FIG. 10C, FIG. 11C, and FIG. 12C show distortion (DT) at a time of focusing to an object point at infinity.

FIG. 7D, FIG. 8D, FIG. 9D, FIG. 10D, FIG. 11D, and FIG. 12D show chromatic aberration of magnification (CC) at a time of focusing to an object point at infinity.

FIG. 7E, FIG. 8E, FIG. 9E, FIG. 10E, FIG. 11E, and FIG. 12E show spherical aberration (SA) at a time of focusing to an object point at a close distance.

FIG. 7F, FIG. 8F, FIG. 9F, FIG. 10F, FIG. 11F, and FIG. 12F show astigmatism (AS) at a time of focusing to an object point at a close distance.

FIG. 7G, FIG. 8G, FIG. 9G, FIG. 10G, FIG. 11G, and FIG. 12G show distortion (DT) at a time of focusing to an object point at a close distance.

FIG. 7H, FIG. 8H, FIG. 9H, FIG. 10H, FIG. 11H, and FIG. 12H show chromatic aberration of magnification (CC) at a time of focusing to an object point at a close distance.

An image forming lens system of Example 1 includes in order from the object side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, and a third lens unit G3 having a positive refractive power. An aperture stop S is disposed between the first lens unit G1 and the second lens unit G2.

The first lens unit G1 includes a positive meniscus lens L1 having a convex surface facing toward the object side, a positive meniscus lens L2 having a convex surface facing toward the object side, a negative meniscus lens L3 having a convex surface facing toward the object side, a positive meniscus lens L4 having a convex surface facing toward the object side, a biconvex positive lens L5, a biconcave negative lens L6, a biconvex positive lens L7, and a biconcave negative lens L8.

Here, the negative meniscus lens L3 and the positive meniscus lens L4 are cemented together. The biconvex positive lens L5 and the biconcave negative lens L6 are cemented together. The biconvex positive lens L7 and the biconcave negative lens L8 are cemented together.

A front-side lens unit includes the positive meniscus lens L1 and the positive meniscus lens L2. A rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit. The first rear-side lens unit includes the negative meniscus lens L3, the positive meniscus lens L4, the biconvex positive lens L5, and the biconcave negative lens L6. The second rear-side lens unit includes the biconvex positive lens L7 and the biconcave negative lens L8.

The second lens unit G2 includes a biconvex positive lens L9 and a biconcave negative lens L10. Here, the biconvex positive lens L9 and the biconcave negative lens L10 are cemented together.

The third lens unit G3 includes a negative meniscus lens L11 having a convex surface facing toward the object side, a biconvex positive lens L12, a biconvex positive lens L13, a biconcave negative lens L14, a biconcave negative lens L15, a biconvex positive lens L16, a negative meniscus lens L17 having a convex surface facing toward the image side, a biconvex positive lens L18, and a negative meniscus lens L19 having a convex surface facing toward the image side.

Here, the negative meniscus lens L11 and the biconvex positive lens L12 are cemented together. The biconvex positive lens L13 and the biconcave negative lens L14 are cemented together. The biconvex positive lens L16 and the negative meniscus lens L17 are cemented together. The biconvex positive lens L18 and the negative meniscus lens L19 are cemented together.

An object-side sub-lens unit includes the negative meniscus lens L11 and the biconvex positive lens L12. A camera shake correction lens unit includes the biconvex positive lens L13, the biconcave negative lens L14, and the biconcave negative lens L15. An image-side sub-lens unit includes the biconvex positive lens L16, the negative meniscus lens L17, the biconvex positive lens L18, and the negative meniscus lens L19.

At a time of focusing from an object at infinity to an object at a close distance, the second lens unit G2 moves to the image side. At a time of camera shake correction, the camera shake correction lens unit moves in a direction vertical to the optical axis.

A diffractive optical element DOE is provided on the image-side surface of the positive meniscus lens L2. The diffractive optical element DOE includes two optical members having a convex surface facing toward the object side. A diffractive surface is formed at the cemented surface of the two optical members.

An image forming lens system of Example 2 includes in order from the object side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, and a third lens unit G3 having a positive refractive power. An aperture stop S is disposed between the first lens unit G1 and the second lens unit G2.

The first lens unit G1 includes a positive meniscus lens L1 having a convex surface facing toward the object side, a positive meniscus lens L2 having a convex surface facing toward the object side, a negative meniscus lens L3 having a convex surface facing toward the object side, a positive meniscus lens L4 having a convex surface facing toward the object side, a biconvex positive lens L5, a biconcave negative lens L6, a biconvex positive lens L7, and a biconcave negative lens L8.

Here, the negative meniscus lens L3 and the positive meniscus lens L4 are cemented together. The biconvex positive lens L5 and the biconcave negative lens L6 are cemented together. The biconvex positive lens L7 and the biconcave negative lens L8 are cemented together.

A front-side lens unit includes the positive meniscus lens L1 and the positive meniscus lens L2. A rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit. The first rear-side lens unit includes the negative meniscus lens L3, the positive meniscus lens L4, the biconvex positive lens L5, and the biconcave negative lens L6. The second rear-side lens unit includes the biconvex positive lens L7 and the biconcave negative lens L8.

The second lens unit G2 includes a biconvex positive lens L9 and a biconcave negative lens L10. Here, the biconvex positive lens L9 and the biconcave negative lens L10 are cemented together.

The third lens unit G3 includes a negative meniscus lens L11 having a convex surface facing toward the object side, a biconvex positive lens L12, a biconvex positive lens L13, a biconcave negative lens L14, a biconcave negative lens L15, a biconvex positive lens L16, a negative meniscus lens L17 having a convex surface facing toward the image side, a biconvex positive lens L18, and a negative meniscus lens L19 having a convex surface facing toward the image side.

Here, the negative meniscus lens L11 and the biconvex positive lens L12 are cemented together. The biconvex positive lens L13 and the biconcave negative lens L14 are cemented together. The biconvex positive lens L16 and the negative meniscus lens L17 are cemented together. The biconvex positive lens L18 and the negative meniscus lens L19 are cemented together.

An object-side sub-lens unit includes the negative meniscus lens L11 and the biconvex positive lens L12. A camera shake correction lens unit includes the biconvex positive lens L13, the biconcave negative lens L14, and the biconcave negative lens L15. An image-side sub-lens unit includes the biconvex positive lens L16, the negative meniscus lens L17, the biconvex positive lens L18, and the negative meniscus lens L19.

At a time of focusing from an object at infinity to an object at a close distance, the second lens unit G2 moves to the image side. At a time of camera shake correction, the camera shake correction lens unit moves in a direction vertical to the optical axis.

A diffractive optical element DOE is provided on the image-side surface of the positive meniscus lens L2. The diffractive optical element DOE includes two optical members having a convex surface facing toward the object side. A diffractive surface is formed at the cemented surface of the two optical members.

An image forming lens system of Example 3 includes in order from the object side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, and a third lens unit G3 having a positive refractive power. An aperture stop S is disposed between the first lens unit G1 and the second lens unit G2.

The first lens unit G1 includes a positive meniscus lens L1 having a convex surface facing toward the object side, a positive meniscus lens L2 having a convex surface facing toward the object side, a negative meniscus lens L3 having a convex surface facing toward the object side, a positive meniscus lens L4 having a convex surface facing toward the object side, a biconvex positive lens L5, a biconcave negative lens L6, a biconvex positive lens L7, and a biconcave negative lens L8.

Here, the negative meniscus lens L3 and the positive meniscus lens L4 are cemented together. The biconvex positive lens L5 and the biconcave negative lens L6 are cemented together. The biconvex positive lens L7 and the biconcave negative lens L8 are cemented together.

A front-side lens unit includes the positive meniscus lens L1 and the positive meniscus lens L2. A rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit. The first rear-side lens unit includes the negative meniscus lens L3, the positive meniscus lens L4, the biconvex positive lens L5, and the biconcave negative lens L6. The second rear-side lens unit includes the biconvex positive lens L7 and the biconcave negative lens L8.

The second lens unit G2 includes a biconvex positive lens L9 and a biconcave negative lens L10. Here, the biconvex positive lens L9 and the biconcave negative lens L10 are cemented together.

The third lens unit G3 includes a negative meniscus lens L11 having a convex surface facing toward the object side, a biconvex positive lens L12, a biconvex positive lens L13, a biconcave negative lens L14, a biconcave negative lens L15, a biconvex positive lens L16, a negative meniscus lens L17 having a convex surface facing toward the image side, a biconvex positive lens L18, and a negative meniscus lens L19 having a convex surface facing toward the image side.

Here, the negative meniscus lens L11 and the biconvex positive lens L12 are cemented together. The biconvex positive lens L13 and the biconcave negative lens L14 are cemented together. The biconvex positive lens L16 and the negative meniscus lens L17 are cemented together. The biconvex positive lens L18 and the negative meniscus lens L19 are cemented together.

An object-side sub-lens unit includes the negative meniscus lens L11 and the biconvex positive lens L12. A camera shake correction lens unit includes the biconvex positive lens L13, the biconcave negative lens L14, and the biconcave negative lens L15. An image-side sub-lens unit includes the biconvex positive lens L16, the negative meniscus lens L17, the biconvex positive lens L18, and the negative meniscus lens L19.

At a time of focusing from an object at infinity to an object at a close distance, the second lens unit G2 moves to the image side. At a time of camera shake correction, the camera shake correction lens unit moves in a direction vertical to the optical axis.

A diffractive optical element DOE is provided on the image-side surface of the positive meniscus lens L2. The diffractive optical element DOE includes two optical members having a convex surface facing toward the object side. A diffractive surface is formed at the cemented surface of the two optical members.

An image forming lens system of Example 4 includes in order from the object side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, and a third lens unit G3 having a positive refractive power. An aperture stop S is disposed between the first lens unit G1 and the second lens unit G2.

The first lens unit G1 includes a biconvex positive lens L1, a positive meniscus lens L2 having a convex surface facing toward the object side, a negative meniscus lens L3 having a convex surface facing toward the object side, a positive meniscus lens L4 having a convex surface facing toward the object side, a biconvex positive lens L5, a biconcave negative lens L6, and a positive meniscus lens L7 having a convex surface facing toward the object side.

Here, the negative meniscus lens L3 and the positive meniscus lens L4 are cemented together. The biconvex positive lens L5, the biconcave negative lens L6, and the positive meniscus lens L7 are cemented together.

A front-side lens unit includes the biconvex positive lens L1 and the positive meniscus lens L2. A rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit. The first rear-side lens unit includes the negative meniscus lens L3 and the positive meniscus lens L4. The second rear-side lens unit includes the biconvex positive lens L5, the biconcave negative lens L6, and the positive meniscus lens L7.

The second lens unit G2 includes a biconvex positive lens L8 and a biconcave negative lens L9. Here, the biconvex positive lens L8 and the biconcave negative lens L9 are cemented together.

The third lens unit G3 includes a negative meniscus lens L10 having a convex surface facing toward the object side, a biconvex positive lens L11, a positive meniscus lens L12 having a convex surface facing toward the image side, a biconcave negative lens L13, a biconcave negative lens L14, a biconvex positive lens L15, a negative meniscus lens L16 having a convex surface facing toward the image side, a biconvex positive lens L17, and a negative meniscus lens L18 having a convex surface facing toward the image side.

Here, the negative meniscus lens L10 and the biconvex positive lens L11 are cemented together. The positive meniscus lens L12 and the biconcave negative lens L13 are cemented together. The biconvex positive lens L15 and the negative meniscus lens L16 are cemented together. The biconvex positive lens L17 and the negative meniscus lens L18 are cemented together.

An object-side sub-lens unit includes the negative meniscus lens L10 and the biconvex positive lens L11. A camera shake correction lens unit includes the positive meniscus lens L12, the biconcave negative lens L13, and the biconcave negative lens L14. An image-side sub-lens unit includes the biconvex positive lens L15, the negative meniscus lens L16, the biconvex positive lens L17, and the negative meniscus lens L18.

At a time of focusing from an object at infinity to an object at a close distance, the second lens unit G2 moves to the image side. At a time of camera shake correction, the camera shake correction lens unit moves in a direction vertical to the optical axis.

A diffractive optical element DOE is provided on the image-side surface of the positive meniscus lens L2. The diffractive optical element DOE includes two optical members having a convex surface facing toward the object side. A diffractive surface is formed at the cemented surface of the two optical members.

An image forming lens system of Example 5 includes in order from the object side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, and a third lens unit G3 having a positive refractive power. An aperture stop S is disposed between the first lens unit G1 and the second lens unit G2.

The first lens unit G1 includes a biconvex positive lens L1, a positive meniscus lens L2 having a convex surface facing toward the object side, a negative meniscus lens L3 having a convex surface facing toward the object side, a positive meniscus lens L4 having a convex surface facing toward the object side, a biconvex positive lens L5, a biconcave negative lens L6, a biconvex positive lens L7, and a biconcave negative lens L8.

Here, the negative meniscus lens L3 and the positive meniscus lens L4 are cemented together. The biconvex positive lens L5 and the biconcave negative lens L6 are cemented together. The biconvex positive lens L7 and the biconcave negative lens L8 are cemented together.

A front-side lens unit includes the biconvex positive lens L1 and the positive meniscus lens L2. A rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit. The first rear-side lens unit includes the negative meniscus lens L3, the positive meniscus lens L4, the biconvex positive lens L5, and the biconcave negative lens L6. The second rear-side lens unit includes the biconvex positive lens L7 and the biconcave negative lens L8.

The second lens unit G2 includes a biconvex positive lens L9 and a biconcave negative lens L10. Here, the biconvex positive lens L9 and the biconcave negative lens L10 are cemented together.

The third lens unit G3 includes a negative meniscus lens L11 having a convex surface facing toward the object side, a biconvex positive lens L12, a biconvex positive lens L13, a biconcave negative lens L14, a biconcave negative lens L15, a biconvex positive lens L16, a negative meniscus lens L17 having a convex surface facing toward the image side, a biconvex positive lens L18, and a negative meniscus lens L19 having a convex surface facing toward the image side.

Here, the negative meniscus lens L11 and the biconvex positive lens L12 are cemented together. The biconvex positive lens L13 and the biconcave negative lens L14 are cemented together. The biconvex positive lens L16 and the negative meniscus lens L17 are cemented together. The biconvex positive lens L18 and the negative meniscus lens L19 are cemented together.

An object-side sub-lens unit includes the negative meniscus lens L11 and the biconvex positive lens L12. A camera shake correction lens unit includes the biconvex positive lens L13, the biconcave negative lens L14, and the biconcave negative lens L15. An image-side sub-lens unit includes the biconvex positive lens L16, the negative meniscus lens L17, the biconvex positive lens L18, and the negative meniscus lens L19.

At a time of focusing from an object at infinity to an object at a close distance, the second lens unit G2 moves to the image side. At a time of camera shake correction, the camera shake correction lens unit moves in a direction vertical to the optical axis.

A diffractive optical element DOE is provided on the image-side surface of the positive meniscus lens L2. The diffractive optical element DOE includes two optical members having a convex surface facing toward the object side. A diffractive surface is formed at the cemented surface of the two optical members.

An image forming lens system of Example 6 includes in order from the object side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, and a third lens unit G3 having a positive refractive power. An aperture stop S is disposed between the first lens unit G1 and the second lens unit G2.

The first lens unit G1 includes a biconvex positive lens L1, a positive meniscus lens L2 having a convex surface facing toward the object side, a biconcave negative lens L3, a biconvex positive lens L4, a biconvex positive lens L5, a biconcave negative lens L6, a biconvex positive lens L7, and a biconcave negative lens L8.

Here, the biconcave negative lens L3 and the biconvex positive lens L4 are cemented together. The biconvex positive lens L5 and the biconcave negative lens L6 are cemented together. The biconvex positive lens L7 and the biconcave negative lens L8 are cemented together.

A front-side lens unit includes the biconvex positive lens L1 and the positive meniscus lens L2. A rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit. The first rear-side lens unit includes the biconcave negative lens L3, the biconvex positive lens L4, the biconvex positive lens L5, and the biconcave negative lens L6. The second rear-side lens unit includes the biconvex positive lens L7 and the biconcave negative lens L8.

The second lens unit G2 includes a biconvex positive lens L9 and a biconcave negative lens L10. Here, the biconvex positive lens L9 and the biconcave negative lens L10 are cemented together.

The third lens unit G3 includes a negative meniscus lens L11 having a convex surface facing toward the object side, a biconvex positive lens L12, a biconvex positive lens L13, a biconcave negative lens L14, a biconcave negative lens L15, a biconvex positive lens L16, a negative meniscus lens L17 having a convex surface facing toward the image side, a biconvex positive lens L18, and a negative meniscus lens L19 having a convex surface facing toward the image side.

Here, the negative meniscus lens L11 and the biconvex positive lens L12 are cemented together. The biconvex positive lens L13 and the biconcave negative lens L14 are cemented together. The biconvex positive lens L16 and the negative meniscus lens L17 are cemented together. The biconvex positive lens L18 and the negative meniscus lens L19 are cemented together.

An object-side sub-lens unit includes the negative meniscus lens L11 and the biconvex positive lens L12. A camera shake correction lens unit includes the biconvex positive lens L13, the biconcave negative lens L14, and the biconcave negative lens L15. An image-side sub-lens unit includes the biconvex positive lens L16, the negative meniscus lens L17, the biconvex positive lens L18, and the negative meniscus lens L19.

At a time of focusing from an object at infinity to an object at a close distance, the second lens unit G2 moves to the image side. At a time of camera shake correction, the camera shake correction lens unit moves in a direction vertical to the optical axis.

A diffractive optical element DOE is provided on the image-side surface of the positive meniscus lens L2. The diffractive optical element DOE includes two optical members having a convex surface facing toward the object side. A diffractive surface is formed at the cemented surface of the two optical members.

Numerical data of each example described above is shown below. In Surface data, r denotes radius of curvature of each lens surface, d denotes a distance between respective lens surfaces, nd denotes a refractive index of each lens for a d-line, νd denotes an Abbe number for each lens and *denotes an diffractive surface.

Further, in Various data, OB denotes a object distance, f denotes a focal length of the entire system, FNO. denotes an F number, ω denotes a half angle of view, LTL denotes a lens total length of the optical system, BF denotes a back focus. Back focus is a unit which is expressed upon air conversion of a distance from a rearmost lens surface to a paraxial image surface. The lens total length is a distance from a frontmost lens surface to the rearmost lens surface plus back focus. Infinite denotes at the time of focusing on an infinite object, Close denotes at the time of focusing on a close object. Unit of OB is mm.

In the present example, the diffractive optical element is described according to the ultra-high index method. In the ultra-high index method, it is possible to replace a zero-thickness diffractive surface with a zero-thickness virtual lens having extremely high refractive index.

A shape of an aspherical surface is defined by the following expression where the direction of the optical axis is represented by z, the direction orthogonal to the optical axis is represented by y, a conical coefficient is represented by K, aspherical surface coefficients are represented by A4, A6, A8, A10, A12 . . . . Z=(y ² /r)/[1+{1−(1+k)(y/r)₂}^(1/2)]+A4y ⁴ +A6y ⁶ +A8y ⁸ +A10y ¹⁰ +A12y ¹²+ . . . .

Further, in the aspherical surface coefficients, ‘e−n’ (where, n is an integral number) indicates ‘10^(−n)’. Moreover, these symbols are commonly used in the following numerical data for each example.

Example 1

Unit mm Surface data Surface no. r d nd νd Object plane ∞ ∞  1 163.684 12.00 1.48749 70.23  2 5575.742 25.32  3 74.210 11.84 1.48749 70.23  4 184.307 0.20 1.63387 23.38  5* 185.848 0.00 1001.00000 −3.45  6 185.850 0.20 1.69534 36.44  7 185.311 39.71  8 190.956 3.20 1.90366 31.32  9 44.154 10.30 1.65412 39.68 10 415.129 0.40 11 51.575 8.70 1.43875 94.93 12 −2153.626 2.80 1.80000 29.84 13 49.390 12.86 14 60.335 8.00 1.75520 27.51 15 −83.970 2.70 1.83481 42.73 16 194.944 5.00 17(Stop) ∞ Variable 18 196.959 2.10 1.80810 22.76 19 −88.327 0.85 1.83481 42.73 20 35.066 Variable 21 34.283 1.00 1.92286 20.88 22 28.189 5.00 1.51633 64.14 23 −128.509 3.60 24 99.226 3.00 1.80810 22.76 25 −48.266 0.90 1.69680 55.53 26 22.063 5.25 27 −35.194 0.90 1.73400 51.47 28 69.278 3.31 29 50.978 6.95 1.73800 32.26 30 −24.481 2.00 1.92286 20.88 31 −62.809 0.20 32 74.818 7.30 1.58144 40.75 33 −26.720 2.36 1.92286 20.88 34 −47.311 33.04 Image plane ∞ Aspherical surface data 5th surface k = 0.000 A4 = −2.98432e−12, A6 = 3.80158e−16, A8 = 1.39670e−21 Various data 2ω 3.17 LTL (in air) 268.58 BF (in air) 33.04 Infinity Close OB ∞ 2.0 f 392.00 FNO. 4.08 3.08 d17 19.00 40.35 d20 28.60 7.25 Unit focal length f1 = 171.45 f2 = −50.38 f3 = 114.01

Example 2

Unit mm Surface data Surface no. r d nd νd Object plane ∞ ∞  1 199.330 14.62 1.48749 70.23  2 2303.706 35.00  3 91.138 16.58 1.48749 70.23  4 279.015 0.20 1.63387 23.38  5* 281.416 0.00 1001.00000 −3.45  6 281.420 0.20 1.69534 36.44  7 283.878 42.73  8 212.568 3.80 1.91082 35.25  9 58.540 12.00 1.43875 94.66 10 336.476 5.00 11 79.080 12.00 1.65412 39.68 12 −215.345 2.80 1.88300 40.76 13 133.776 40.01 14 112.459 6.00 1.67270 32.10 15 −99.320 2.30 1.91082 35.25 16 792.510 5.00 17(Stop) ∞ Variable 18 234.583 2.20 1.80810 22.76 19 −170.666 0.90 1.80610 40.92 20 36.369 Variable 21 32.025 1.00 1.92286 20.88 22 24.052 5.00 1.48749 70.23 23 −142.023 3.60 24 344.791 3.10 1.80810 22.76 25 −43.705 0.90 1.69680 55.53 26 24.836 4.35 27 −50.186 0.90 1.77250 49.60 28 123.153 4.30 29 46.998 6.90 1.74000 28.30 30 −45.412 2.20 1.92286 20.88 31 −83.326 10.30 32 55.703 7.00 1.73800 32.26 33 −33.392 2.20 1.92286 20.88 34 −586.621 33.03 Image plane ∞ Aspherical surface data 5th surface k = 0.000 A4 = −1.18604e−12, A6 = 7.14800e−17, A8 = −2.65430e−21 Various data 2ω 2.55 LTL (in air) 328.58 BF (in air) 33.03 Infinity Close OB ∞ 3.5 f 490.00 FNO. 4.06 3.08 d17 17.00 35.42 d20 25.46 7.05 Unit focal length f1 = 219.67 f2 = −53.83 f3 = 117.06

Example 3

Unit mm Surface data Surface no. r d nd νd Object plane ∞ ∞  1 174.154 11.00 1.48749 70.23  2 2616.234 22.80  3 77.916 11.80 1.48749 70.23  4 168.223 0.20 1.63387 23.38  5* 167.436 0.00 1001.00000 −3.45  6 167.438 0.20 1.69534 36.44  7 167.306 47.59  8 155.128 3.20 1.91082 35.25  9 47.385 11.53 1.49700 81.54 10 332.628 9.02 11 72.379 9.00 1.54072 47.23 12 −156.345 2.80 1.88300 40.76 13 113.020 24.77 14 71.203 5.50 1.62004 36.26 15 −91.917 2.00 1.88300 40.76 16 −416.459 3.00 17(Stop) ∞ Variable 18 657.445 2.08 1.80810 22.76 19 −100.323 0.79 1.71300 53.87 20 30.352 Variable 21 33.984 5.35 1.43875 94.66 22 −92.054 3.87 23 369.045 2.43 1.92286 20.88 24 −53.903 0.89 1.61800 63.40 25 23.158 3.88 26 −36.330 0.89 1.69680 55.53 27 46.784 3.31 28 51.248 8.04 1.74951 35.33 29 −24.677 1.80 1.92286 20.88 30 −80.337 0.30 31 69.108 8.63 1.59551 39.24 32 −25.252 2.30 1.92286 20.88 33 −43.499 34.95 Image plane ∞ Aspherical surface data 5th surface k = 0.000 A4 = −1.88780e−12, A6 = 4.71050e−17, A8 = 3.15678e−20 Various data 2ω 3.12 LTL (in air) 288.10 BF (in air) 34.95 Infinity Close OB ∞ 2.5 f 397.04 FNO. 4.13 3.08 d17 20.39 35.86 d20 23.80 8.33 Unit focal length f1 = 171.93 f2 = −47.04 f3 = 110.78

Example 4

Unit mm Surface data Surface no. r d nd νd Object plane ∞ ∞  1 163.072 13.10 1.48749 70.23  2 −1821.264 3.50  3 80.252 12.99 1.48749 70.23  4 191.699 0.20 1.63387 23.38  5* 191.157 0.00 1001.00000 −3.45  6 191.159 0.20 1.69534 36.44  7 190.621 46.46  8 125.211 3.20 1.80518 25.42  9 40.490 9.90 1.48749 70.23 10 240.471 0.50 11 54.046 8.70 1.48749 70.23 12 −491.637 2.80 1.88300 40.76 13 35.678 7.00 1.80810 22.76 14 129.830 11.77 15(Stop) ∞ Variable 16 398.589 2.10 1.80810 22.76 17 −194.391 0.85 1.83481 42.73 18 43.759 Variable 19 43.766 1.00 1.92286 20.88 20 25.888 5.00 1.58144 40.75 21 −84.479 3.60 22 −3124.458 3.00 1.80810 22.76 23 −39.283 0.90 1.69680 55.53 24 25.634 4.56 25 −53.532 0.90 1.75500 52.32 26 54.729 3.30 27 60.854 7.27 1.76182 26.52 28 −20.797 2.00 1.92286 20.88 29 −93.992 0.20 30 60.584 7.30 1.66680 33.05 31 −29.698 2.20 1.92286 20.88 32 −50.255 33.07 Image plane ∞ Aspherical surface data 5th surface k = 0.000 A4 = −6.15645e−13, A6 = 5.26879e−17, A8 = −4.74049e−20 Various data 2ω 3.15 LTL (in air) 243.62 BF (in air) 33.07 Infinity Close OB ∞ 2.0 f 392.22 FNO. 4.08 3.08 d15 16.50 38.42 d18 29.55 7.63 Unit focal length f1 = 165.93 f2 = −58.39 f3 = 139.72

Example 5

Unit mm Surface data Surface no. r d nd νd Object plane ∞ ∞  1 156.124 13.20 1.48749 70.23  2 −2886.966 8.00  3 76.617 12.90 1.48749 70.23  4 168.575 0.20 1.63387 23.38  5* 169.701 0.00 1001.00000 −3.45  6 169.703 0.20 1.69534 36.44  7 170.852 48.08  8 180.665 3.20 1.80000 29.84  9 40.697 10.00 1.48749 70.23 10 1084.061 0.68 11 54.271 8.69 1.51633 64.14 12 −447.828 2.80 1.80440 39.59 13 46.351 3.70 14 47.570 7.00 1.74077 27.79 15 −115.256 1.70 1.83481 42.73 16 133.131 5.00 17(Stop) ∞ Variable 18 577.099 2.10 1.80810 22.76 19 −145.746 0.85 1.83481 42.73 20 42.582 Variable 21 42.464 1.00 1.92286 20.88 22 25.851 5.00 1.59551 39.24 23 −83.286 3.60 24 2970.173 3.00 1.80810 22.76 25 −35.882 0.90 1.69680 55.53 26 27.588 3.79 27 −62.070 0.90 1.80400 46.58 28 44.217 3.30 29 62.869 7.04 1.76182 26.52 30 −20.662 2.00 1.92286 20.88 31 −98.631 0.20 32 55.218 7.30 1.63980 34.46 33 −27.865 2.20 1.92286 20.88 34 −45.752 33.04 Image plane ∞ Aspherical surface data 5th surface k = 0.000 A4 = −1.79878e−12, A6 = 2.33079e−16, A8 = −1.40368e−20 Various data 2ω 3.15 LTL (in air) 248.58 BF (in air) 33.04 Infinity Close OB ∞ 2.0 f 391.90 FNO. 4.08 3.08 d17 18.00 39.47 d20 29.01 7.54 Unit focal length f1 = 166.98 f2 = −54.51 f3 = 131.78

Example 6

Unit mm Surface data Surface no. r d nd νd Object plane ∞ ∞  1 211.558 10.70 1.48749 70.23  2 −8057.967 57.52  3 84.860 11.90 1.48749 70.23  4 266.991 0.20 1.63387 23.38  5* 271.245 0.00 1001.00000 −3.45  6 271.251 0.20 1.69534 36.44  7 270.593 43.87  8 −1517.345 3.20 1.68893 31.07  9 55.750 10.30 1.48749 70.23 10 −325.599 3.11 11 58.912 8.53 1.51633 64.14 12 −3194.168 2.80 1.72342 37.95 13 49.278 5.23 14 61.445 10.00 1.75520 27.51 15 −60.399 2.65 1.74950 35.28 16 156.687 4.00 17(Stop) ∞ Variable 18 275.605 2.10 1.80810 22.76 19 −116.568 0.85 1.83481 42.73 20 37.923 Variable 21 32.776 1.00 1.92286 20.88 22 24.270 5.00 1.51742 52.43 23 −104.847 3.60 24 323.397 3.00 1.80810 22.76 25 −39.947 0.90 1.69680 55.53 26 25.407 3.94 27 −50.154 0.90 1.77250 49.60 28 50.515 3.30 29 59.037 6.90 1.75520 27.51 30 −22.127 2.00 1.92286 20.88 31 −80.940 0.20 32 60.161 7.92 1.60342 38.03 33 −27.717 2.20 1.92286 20.88 34 −44.602 33.04 Image plane ∞ Aspherical surface data 5th surface k = 0.000 A4 = −4.43936e−12, A6 = 4.71510e−16 Various data 2ω 3.15 LTL (in air) 298.58 BF (in air) 33.04 Infinity Close OB ∞ 2.0 f 391.95 FNO. 4.08 3.08 d17 19.00 40.25 d20 28.52 7.28 Unit focal length f1 = 169.75 f2 = −52.06 f3 = 119.34

The values of the conditional expressions in Examples are listed below. The value of Conditional Expression (1-1), (1-2), (1-3), and (1-4) are the same as the value of Conditional Expression (1). The value of Conditional Expression (2-1) is the same as the value of Conditional Expression (2)

Example1 Example2 Example3 (1) ΔGFGR/f 0.101 0.087 0.120 (2) ΔGFFGR1/f 0.228 0.223 0.236 (3) νdGFave 70.230 70.230 70.230 (4) νdGFmax 70.230 70.230 70.230 (5) DGR1GR2/f 0.033 0.082 0.062 (6) fGF/f 0.380 0.359 0.425 (7) fL1/fGF 2.318 2.542 2.267 (8) fGF/fGR1 −1.423 −0.734 −1.133 (9) |fG2/f| 0.129 0.110 0.118 (10) |MGG2B² × (MGG2² − 1)| 4.700 4.450 4.823 (13) |MGISB × (MGIS − 1)| 2.000 2.000 2.000 (14) DGFairmax/DGF 0.511 0.526 0.496 (15) ΔGFGR/fGF 0.266 0.243 0.282 (16) νdLp1 70.230 70.230 70.230 (17) |fG1/fG2| 3.403 4.081 3.655 (18) ΣdGF/fGF 0.332 0.379 0.273 Example4 Example5 Example6 (1) ΔGFGR/f 0.118 0.123 0.112 (2) ΔGFFGR1/f 0.195 0.211 0.317 (3) νdGFave 70.230 70.230 70.230 (4) νdGFmax 70.230 70.230 70.230 (5) DGR1GR2/f 0.001 0.009 0.013 (6) fGF/f 0.369 0.368 0.433 (7) fL1/fGF 2.123 2.108 2.492 (8) fGF/fGR1 −0.485 −1.660 −1.364 (9) |fG2/f| 0.149 0.139 0.133 (10) |MGG2B² × (MGG2² − 1)| 4.705 4.753 4.699 (13) |MGISB × (MGIS − 1)| 2.000 2.000 2.000 (14) DGFairmax/DGF 0.117 0.232 0.714 (15) ΔGFGR/fGF 0.321 0.333 0.258 (16) νdLp1 70.230 70.230 70.230 (17) |fG1/fG2| 2.842 3.063 3.260 (18) ΣdGF/fGF 0.207 0.239 0.474

FIG. 13 is a cross-sectional view of a single-lens mirrorless camera as an electronic image pickup apparatus. In FIG. 13, a photographic optical system 2 is disposed inside a lens barrel of a single-lens mirrorless camera 1. A mount portion 3 enables the photographic optical system 2 to be detachable from a body of the single-lens mirrorless camera 1. As the mount portion 3, a mount such as a screw-type mount and a bayonet-type mount is to be used. In this example, a bayonet-type mount is used. Moreover, an image pickup element surface 4 and a back monitor 5 are disposed in the body of the single-lens mirrorless camera 1. As an image pickup element, an element such as a small-size CCD (charge coupled device) or a CMOS (complementary metal-oxide semiconductor) is to be used.

Moreover, as the photographic optical system 2 of the single-lens mirrorless camera 1, the image forming lens system described in any one of the examples from the first example to the sixth example is to be used.

FIG. 14 and FIG. 15 are conceptual diagrams of an arrangement of the image pickup apparatus. FIG. 14 is a front perspective view of a digital camera 40 as the image pickup apparatus, and FIG. 15 is a rear perspective view of the digital camera 40. The image forming lens system according to the present example is used in a photographic optical system 41 of the digital camera 40.

The digital camera 40 according to the present embodiment includes the photographic optical system 41 which is positioned in a photographic optical path 42, a shutter button 45, and a liquid-crystal display monitor 47. As the shutter button 45 disposed on an upper portion of the digital camera 40 is pressed, in conjunction with the pressing of the shutter button 45, photography is carried out by the photographic optical system 41 such as the image forming lens system according to the first example. An object image which is formed by the photographic optical system 41 is formed on an image pickup element (photoelectric conversion surface) which is provided near an image forming surface. The object image which has been received optically by the image pickup element is displayed on the liquid-crystal display monitor 47 which is provided to a rear surface of the camera, as an electronic image by a processing means. Moreover, it is possible to record the electronic image which has been photographed, in a storage means.

FIG. 16 is a structural block diagram of an internal circuit of main components of the digital camera 40. In the following description, the processing means described above includes for instance, a CDS/ADC section 24, a temporary storage memory 117, and an image processing section 18, and a storage means consists of a storage medium section 19 for example.

As shown in FIG. 16, the digital camera 40 includes an operating section 12, a control section 13 which is connected to the operating section 12, the temporary storage memory 17 and an imaging drive circuit 16 which are connected to a control-signal output port of the control section 13, via a bus 14 and a bus 15, the image processing section 18, the storage medium section 19, a display section 20, and a set-information storage memory section 21.

The temporary storage memory 17, the image processing section 18, the storage medium section 19, the display section 20, and the set-information storage memory section 21 are structured to be capable of mutually inputting and outputting data via a bus 22. Moreover, the CCD 49 and the CDS/ADC section 24 are connected to the imaging drive circuit 16.

The operating section 12 includes various input buttons and switches, and informs the control section 13 of event information which is input from outside (by a user of the digital camera) via these input buttons and switches. The control section 13 is a central processing unit (CPU), and has a built-in computer program memory which is not shown in the diagram. The control section 13 controls the entire digital camera 40 according to a computer program stored in this computer program memory.

The CCD 49 is driven and controlled by the imaging drive circuit 16, and which converts an amount of light for each pixel of the object image formed by the photographic optical system 41 to an electric signal, and outputs to the CDS/ADC section 24.

The CDS/ADC section 24 is a circuit which amplifies the electric signal which is input from the CCD 49, and carries out analog/digital conversion, and outputs to the temporary storage memory 17 image raw data (Bayer data, hereinafter called as ‘RAW data’) which is only amplified and converted to digital data.

The temporary storage memory 17 is a buffer which includes an SDRAM (Synchronous Dynamic Random Access Memory) for example, and is a memory device which stores temporarily the RAW data which is output from the CDS/ADC section 24. The image processing section 18 is a circuit which reads the RAW data stored in the temporary storage memory 17, or the RAW data stored in the storage medium section 19, and carries out electrically various image-processing including the distortion correction, based on image-quality parameters specified by the control section 13.

The storage medium section 19 is a recording medium in the form of a card or a stick including a flash memory for instance, detachably mounted. The storage medium section 19 records and maintains the RAW data transferred from the temporary storage memory 17 and image data subjected to image processing in the image processing section 18 in the card flash memory and the stick flash memory.

The display section 20 includes the liquid-crystal display monitor, and displays photographed RAW data, image data and operation menu on the liquid-crystal display monitor. The set-information storage memory section 21 includes a ROM section in which various image quality parameters are stored in advance, and a RAM section which stores image quality parameters which are selected by an input operation on the operating section 12, from among the image quality parameters which are read from the ROM section.

The present invention can have various modified examples without departing from the scope of the invention. Moreover, shapes of lenses and the number of lenses are not necessarily restricted to the shapes and the number of lenses indicated in the examples. In the examples described heretofore, the cover glass may not be disposed necessarily. A lens that is not shown in the diagrams of the examples described above, and that does not have a refractive power practically may be disposed in a lens unit or outside the lens unit.

A lens may be formed of a single material or may be formed of a plurality of materials. Examples of the lens formed of a plurality of glass materials include cemented lenses as well as hybrid lenses and diffractive optical elements as described above. Diffractive optical elements exist, for example, in two forms. In the first form, a diffraction-effect surface is formed on the surface of a lens formed of a single material. In the second form, another material is formed on the surface of a lens formed of a single material and a diffraction-effect surface is formed on the surface of the other material.

The present invention can provide an image forming lens system having excellent portability and in which aberration is corrected favorably, and an image pickup apparatus including the same.

As described above, the present invention is suitable for an image forming lens system having excellent portability and in which aberration is corrected favorably, and for an image pickup apparatus having excellent portability and capable of producing a high-quality image. 

What is claimed is:
 1. An image forming lens system comprising, in order from an object side to an image side: a first lens unit having a positive refractive power; a second lens unit having a negative refractive power; and a third lens unit, wherein: the first lens unit includes a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween in order from the object side, the second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing, the third lens unit includes a positive lens element, each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a), the rear-side lens unit includes a negative lens element and a positive lens element, the front-side lens unit includes a diffractive lens component having a diffractive lens surface, the diffractive lens component has a positive refractive power, and following Conditional Expression (1) is satisfied: −0.7≤f/fLens  (a) 0.015≤ΔGFGR/f≤0.25  (1) where f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, fLens is a focal length of each lens element in the front-side lens unit, ΔGFGR is an on-axis air space from an image-side surface in the front-side lens unit to an object-side surface in the rear-side lens unit, a lens component means a lens in which only two surfaces, namely, an object-side surface and an image-side surface are effective surfaces in contact with air, and a lens element is a lens having two effective transmission surfaces, namely, an object-side surface and an image-side surface and having no other effective transmission surface between the two effective transmission surfaces.
 2. The image forming lens system according to claim 1, wherein each lens component in the front-side lens unit is the diffractive lens component having a positive refractive power or a single lens having a positive refractive power.
 3. The image forming lens system according to claim 1, wherein: the front-side lens unit is composed of all lens elements that are positioned closer to the object side than a predetermined negative lens element, the predetermined negative lens element is a negative lens element disposed closest to the object side, of negative lens elements that satisfy following Conditional Expression (a-1), and the negative lens element of the rear-side lens unit is the predetermined negative lens element: 0.4≤|f/fLn|  (a-1) where f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and fLn is a focal length of the predetermined negative lens element.
 4. An image forming lens system comprising, in order from an object side to an image side: a first lens unit having a positive refractive power; a second lens unit having a negative refractive power; and a third lens unit, wherein: the first lens unit includes a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween in order from the object side, the second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing, the third lens unit includes a positive lens element, each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a), the rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit, the first rear-side lens unit includes a negative lens element and a positive lens element, the second rear-side lens unit includes a positive lens element, the front-side lens unit includes a diffractive lens component having a diffractive lens surface, the diffractive lens component has a positive refractive power, and following Conditional Expression (2) is satisfied: −0.7≤f/fLens  (a) 0.1≤ΔGFFGR1/f≤0.5  (2) where f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, fLens is a focal length of each lens element in the front-side lens unit, ΔGFFGR1 is an on-axis distance from an object-side surface in the front-side lens unit to an object-side surface in the first rear-side lens unit, a lens component means a lens in which only two surfaces, namely, an object-side surface and an image-side surface are effective surfaces in contact with air, and a lens element is a lens having two effective transmission surfaces, namely, an object-side surface and an image-side surface and having no other effective transmission surface between the two effective transmission surfaces.
 5. The image forming lens system according to claim 4, wherein each lens component in the front-side lens unit is the diffractive lens component having a positive refractive power or a single lens having a positive refractive power.
 6. The image forming lens system according to claim 4, wherein: the front-side lens unit is composed of all lens elements that are positioned closer to the object side than a predetermined negative lens element, the predetermined negative lens element is a negative lens element disposed closest to the object side, of negative lens elements that satisfy following Conditional Expression (a-1), and the negative lens element of the rear-side lens unit is the predetermined negative lens element: 0.4≤|f/fLn|  (a-1) where f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and fLn is a focal length of the predetermined negative lens element.
 7. An image forming lens system comprising, in order from an object side to an image side: a first lens unit having a positive refractive power; a second lens unit having a negative refractive power; and a third lens unit, wherein: the first lens unit includes a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween in order from the object side, the second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing, the third lens unit includes a positive lens element, the rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit, the first rear-side lens unit includes a negative lens element and a positive lens element, the second rear-side lens unit includes a positive lens element, the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface, the diffractive lens component is disposed closest to the image side in the front-side lens unit and includes a diffractive lens element closest to the image side, and following Conditional Expression (1-1) is satisfied: 0.015≤ΔGFGR1Do/f≤0.3  (1-1) where f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, ΔGFGR1Do is an on-axis air space from an image-side surface of the diffractive lens element to an object-side surface in the first rear-side lens unit, a lens component means a lens in which only two surfaces, namely, an object-side surface and an image-side surface are effective surfaces in contact with air, and a lens element is a lens having two effective transmission surfaces, namely, an object-side surface and an image-side surface and having no other effective transmission surface between the two effective transmission surfaces.
 8. The image forming lens system according to claim 7, wherein: the front-side lens unit is composed of all lens elements that are positioned closer to the object side than a predetermined negative lens element, the predetermined negative lens element is a negative lens element disposed closest to the object side, of negative lens elements that satisfy following Conditional Expression (a-1): 0.4≤|f/fLn|  (a-1) where f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and fLn is a focal length of the predetermined negative lens element.
 9. The image forming lens system according to claim 7, wherein: each lens component in the front-side lens unit is the diffractive lens component or a single lens having a positive refractive power, and the diffractive lens component has a positive refractive power.
 10. An image forming lens system comprising, in order from an object side to an image side: a first lens unit having a positive refractive power; a second lens unit having a negative refractive power; and a third lens unit, wherein: the first lens unit includes a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween in order from the object side, the second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing, the third lens unit includes a positive lens element, each lens element in the front-side lens unit is a lens element that satisfies following Conditional Expression (a), the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface, the diffractive lens component is disposed closest to the image side in the front-side lens unit and includes a diffractive lens element closest to the image side, and following Conditional Expression (1-2) is satisfied: −0.7≤f/fLens  (a) 0.02≤ΔGFGRDo/f≤0.3  (1-2) where f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, fLens is a focal length of each lens element in the front-side lens unit, ΔGFGRDo is an on-axis air space from an image-side surface of the diffractive lens element to an object-side surface in the rear-side lens unit, a lens component means a lens in which only two surfaces, namely, an object-side surface and an image-side surface are effective surfaces in contact with air, and a lens element is a lens having two effective transmission surfaces, namely, an object-side surface and an image-side surface and having no other effective transmission surface between the two effective transmission surfaces.
 11. The image forming lens system according to claim 10, wherein: the front-side lens unit is composed of all lens elements that are positioned closer to the object side than a predetermined negative lens element, the predetermined negative lens element is a negative lens element disposed closest to the object side, of negative lens elements that satisfy following Conditional Expression (a-1): 0.4≤|f/fLn|  (a-1) where f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and fLn is a focal length of the predetermined negative lens element.
 12. An image forming lens system comprising, in order from an object side to an image side: a first lens unit having a positive refractive power; a second lens unit having a negative refractive power; and a third lens unit, wherein: the first lens unit includes a front-side lens unit having a positive refractive power and a rear-side lens unit with an air space interposed therebetween in order from the object side, the second lens unit moves by changing respective air spaces on the object side and the image side at a time of focusing, the third lens unit includes a positive lens element, the front-side lens unit includes a positive lens component disposed closest to the object side and a diffractive lens component having a diffractive lens surface, the diffractive lens component is disposed closest to the image side in the front-side lens unit, the rear-side lens unit includes a positive lens and a negative lens, an air space between the diffractive lens component and the rear-side lens unit is largest of air spaces in the first lens unit, the image forming lens system further comprises an aperture stop disposed closer to the object side than the second lens unit to limit on-axis light flux, and following Conditional Expression (1-3) is satisfied: 0.01≤ΔGFDoGRmax/f≤0.2  (1-3) where f is a longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, ΔGFDoGRmax is a largest on-axis air space of on-axis air spaces from an image-side surface of the diffractive lens component to an object-side surface in the rear-side lens unit, a lens component means a lens in which only two surfaces, namely, an object-side surface and an image-side surface are effective surfaces in contact with air, and a lens element is a lens having two effective transmission surfaces, namely, an object-side surface and an image-side surface and having no other effective transmission surface between the two effective transmission surfaces.
 13. The image forming lens system according to claim 12, wherein: in the front-side lens unit, a positive lens element is disposed closest to the object side, and the diffractive lens component is disposed closest to the image side, each lens component in the front-side lens unit is the diffractive lens component or a single lens having a positive refractive power, and the diffractive lens component has a positive refractive power.
 14. The image forming lens system according to claim 12, wherein: the front-side lens unit is composed of all lens elements that are positioned closer to the object side than a predetermined negative lens element, the predetermined negative lens element is a negative lens element disposed closest to the object side, of negative lens elements that satisfy following Conditional Expression (a-1′), and the negative lens element of the rear-side lens unit is the predetermined negative lens element: 0.5≤|f/fLn|  (a-1′) where f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and fLn is a focal length of the predetermined negative lens element.
 15. The image forming lens system according to claim 1, wherein: the rear-side lens unit includes a first rear-side lens unit and a second rear-side lens unit with an air space interposed therebetween, the first rear-side lens unit includes a negative lens element and a positive lens element, and the second rear-side lens unit includes a positive lens element.
 16. The image forming lens system according to claim 4, wherein the first rear-side lens unit has a negative refractive power.
 17. The image forming lens system according to claim 4, wherein the second rear-side lens unit has a positive refractive power.
 18. The image forming lens system according to claim 4, wherein the following Conditional Expression (1) is satisfied: 0.015≤ΔGFGR/f≤0.25  (1) where f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and ΔGFGR is an on-axis air space from an image-side surface in the front-side lens unit to an object-side surface in the rear-side lens unit.
 19. The image forming lens system according to claim 7, wherein the following Conditional Expression (2) is satisfied: 0.1≤ΔGFFGR1/f≤0.5  (2) where f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, ΔGFFGR1 is an on-axis distance from an object-side surface in the front-side lens unit to an object-side surface in the first rear-side lens unit.
 20. The image forming lens system according to claim 1, wherein: the diffractive lens component is disposed closest to the image side in the front-side lens unit and includes a diffractive lens element closest to the image side, and following Conditional Expression (2-1) is satisfied: 0.10≤ΔGFFGR1Do/f≤0.5  (2-1) where f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, ΔGFFGR1Do is an on-axis distance from an object-side surface of the diffractive lens element to an object-side surface in the first rear-side lens unit, a lens component means a lens in which only two surfaces, namely, an object-side surface and an image-side surface are effective surfaces in contact with air, and a lens element is a lens having two effective transmission surfaces, namely, an object-side surface and an image-side surface and having no other effective transmission surface between the two effective transmission surfaces.
 21. The image forming lens system according to claim 1, wherein the following Conditional Expression (3) is satisfied: 50≤vdGFave  (3) where vdGFave is an average Abbe number of positive lens elements in the front-side lens unit.
 22. The image forming lens system according to claim 1, wherein the following Conditional Expression (4) is satisfied: 50≤vdGFmax  (4) where vdGFmax is a largest Abbe number of Abbe numbers of positive lens elements in the front-side lens unit.
 23. The image forming lens system according to claim 4, wherein the following Conditional Expression (5) is satisfied: 0.008≤DGR1GR2/f≤0.3  (5), where, DGR1GR2 is an axial air space between the first rear-side lens unit and the second rear-side lens unit; and f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance.
 24. The image forming lens system according to claim 1, wherein the front-side lens unit has a plurality of the positive lens elements.
 25. The image forming lens system according to claim 1, wherein the following Conditional Expression (6) is satisfied: 0.2≤fGF/f≤0.8  (6), where, fGF is a focal length of the front-side lens unit; and f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance.
 26. The image forming lens system according to claim 1, wherein: a first lens element is disposed closest to the object, and following Conditional Expression (7) is satisfied: 1.6≤fL1/fGF≤5.0  (7), where, fL1 is a focal length of the first lens element; and fGF is a focal length of the front-side lens unit.
 27. The image forming lens system according to claim 4, wherein the following Conditional Expression (8) is satisfied: −3.0≤fGF/fGR1≤0.1  (8), where, fGF is a focal length of the front-side lens unit; and fGR1 is a focal length of the first rear-side lens unit.
 28. The image forming lens system according to claim 1, wherein the following Conditional Expression (9) is satisfied: 0.06≤|fG2/f|≤0.25  (9), where, fG2 is a focal length of the second lens unit; and f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance.
 29. The image forming lens system according to claim 1, wherein the following Conditional Expression (10) is satisfied: 3.0≤|MGG2B ²×(MGG2²−1)|≤6.5  (10), where, MGG2B is a lateral magnification of a first rear-side lens system; MGG2 is a lateral magnification of the second lens unit; the lateral magnification is a lateral magnification at the time of focusing on an infinite object; and the first rear-side lens system is a lens system comprising all of lenses positioned closer to an image side than the second lens unit.
 30. The image forming lens system according to claim 1, wherein the second lens unit includes two or less lens elements.
 31. The image forming lens system according to claim 1, wherein the second lens unit includes a negative lens element and a positive lens element.
 32. The image forming lens system according to claim 1, wherein: the third lens unit has a camera shake-correction lens unit, and the camera shake-correction lens unit moves in a direction vertical to an optical axis.
 33. The image forming lens system according to claim 1, wherein the third lens unit has: an object-side sub-lens unit having a positive refractive power, a camera shake-correction lens unit having a negative refractive power, and an image-side sub-lens unit having a positive refractive power.
 34. The image forming lens system according to claim 1, wherein the aperture stop is disposed on the object side of the second lens unit.
 35. The image forming lens system according to claim 1, wherein the aperture stop is disposed between the first lens unit and the second lens unit.
 36. The image forming lens system according to claim 1, wherein the aperture stop is disposed between the first rear-side lens unit and the second lens unit.
 37. The image forming lens system according to claim 1, wherein the following Conditional Expression (14) is satisfied: 0.19≤DGFairmax/DGF≤1.0  (14), where, DGFairmax is a largest axial air space, of axial air spaces in the front-side lens unit; and DGF is an axial thickness of the front-side lens unit.
 38. The image forming lens system according to claim 1, wherein the following Conditional Expression (1-4) is satisfied: 0.01≤ΔGFGRmax/f≤0.2  (1-4) where f is the longest focal length of the entire image forming lens system at a time of focusing to a furthest distance, and ΔGFGRmax is a largest on-axis air space of on-axis air spaces from an image-side surface in the front-side lens unit to an object-side surface in the rear-side lens unit.
 39. The image forming lens system according to claim 1, wherein the following Conditional Expression (15) is satisfied: 0.05≤ΔGFGR/fGF≤0.4  (15) where ΔGFGR is an on-axis air space from an image-side surface in the front-side lens unit to an object-side surface in the rear-side lens unit, and fGF is a focal length of the front-side lens unit.
 40. The image forming lens system according to claim 1, wherein the following Conditional Expression (16) is satisfied: 50≤vdLp1  (16), where, vdLp1 is an Abbe number of a positive lens element positioned closest to the object.
 41. The image forming lens system according to claim 1, wherein the front-side lens unit comprises two positive lens elements.
 42. The image forming lens system according to claim 1, wherein the first rear-side lens unit has at least two negative lenses.
 43. The image forming lens system according to claim 1, wherein the following Conditional Expression (17) is satisfied: 1.5≤|fG1/fG2|≤6.5  (17), where, fG1 is a focal length of the first lens unit; and fG2 is a focal length of the second lens unit.
 44. The image forming lens system according to claim 1, wherein a lens unit that moves in an optical axis direction is the second lens unit alone.
 45. The image forming lens system according to claim 1, wherein the third lens unit includes a positive lens element and a negative lens element.
 46. The image forming lens system according to claim 1, wherein the front-side lens unit includes at least one of a lens element and a lens component, the lens element and the lens component having a positive refractive power.
 47. The image forming lens system according to claim 1, wherein the following Conditional Expression (18) is satisfied: 0.15≤τdGF/fGF≤0.7  (18) where ΣdGF is a total thickness of the front-side lens unit, and fGF is a focal length of the front-side lens unit.
 48. The image forming lens system according to claim 1, wherein: the front-side lens unit includes a positive lens component disposed closest to the object side and the diffractive lens component, and the diffractive lens component is disposed closest to the image side in the front-side lens unit and includes a diffractive lens element closest to the image side.
 49. The image forming lens system according to claim 4, wherein a distance between the front-side lens unit and the first rear-side lens unit and a distance between the first rear-side lens unit and the second rear-side lens unit are always constant.
 50. The image forming lens system according to claim 1, wherein the front-side lens unit includes a lens element located closer to the object side than the predetermined negative lens element that satisfies following Conditional Expression (b): 0.02≤DNx/φenp  (b) where DNx is a thickness on an optical axis of the predetermined negative lens element, and φenp is a maximum diameter of an entrance pupil of the image forming lens system in a state of achieving a longest focal length at a time of focusing to a furthest distance.
 51. The image forming lens system according to claim 1, wherein a total length of an optical system is fixed in all states.
 52. The image forming lens system according to claim 1, wherein a lens unit located closest to the image side is fixed in all states.
 53. The image forming lens system according to claim 1, wherein an aperture stop is fixed in position in all states.
 54. The image forming lens system according to claim 1, wherein the first lens unit is fixed in all states.
 55. The image forming lens system according to claim 1, wherein the front-side lens unit includes a lens located closer to the object side than a predetermined negative lens that satisfies following Conditional Expression (c): 0.007≤DNx/LTLmin  (c) where DNx is a thickness on an optical axis of the predetermined negative lens element, and LTLmin is a minimum total length of the image forming lens system.
 56. An image pickup apparatus comprising: an optical system; and an image pickup element having an image plane and converting an image formed on the image plane by the optical system to an electrical signal, wherein the optical system is the image forming lens system according to claim
 1. 